IMMUNOLOGY AND MEDICAL MICROBIOLOGY
Immune system
Dr. Kusum R. Gupta (Sharma) Senior Lecturer
Department of Microbiology Ram Lal Anand College (Univ. of Delhi)
Benito Juarez Road New Delhi-110021 CONTENTS
Immunology from Early Writings Cellular Immunity
General Concepts of Immune System Early Theories of Immunity
Antigen Recognition by B and T Cells Cells and Organs of the Immune System
Lymphoid Cells Granulocytic Cells Dendritic Cells Mononuclear Cells Organs of the Immune System
Primary Lymphoid Organs Bone Marrow
Thymus
Secondary Lymphoid Organs Lymph Nodes
Spleen
Mucosal associated Lymphoid Tissues Cutaneous associated Lymphoid Tissue Innate Immunity
Acquired Immunity Humoral Immunity Cell Mediated Immunity
Keywords
B lymphocytes; B cell receptor (BCR); T lymphocytes; T cell receptor (TCR); Epitopes; Major histocompatibility complex (MHC); Antigen presenting cell (APC); T helper (Th )cell; T cytotoxic (Tc )cell; T suppressor (Ts ) cell;
Cytotoxic T lymphocyte (CTL); Cell mediated immunity (CMI); Humoral immunity; Hematopoietic stem cell (HSC); Hematopoiesis inducing micro (HIM) environment; Colony stimulating factor (CSF); Immunoglobulin (Ig);
Interferon (IFN); Interleukin (IL); Natural killer (NK)cell; Antibody dependent cell mediated cytotoxicity (ADCC);
Tumor necrosis factor (TNF); Mucosal associated lymphoid tissue (MALT)
Historical Background
The word “immunity” was used earliest in the context of being free of the burden of taxes or military conspiration. The history of immunology is slightly more than 100 years old if one considers Louis Pasteur as the “Father of Immunology”, as most do. However, if cellular immunology is considered, the ‘real’ history begins in the late 1950.
Immunology from Early Writings
From early writings, it is clear that primitive man knew about disease and its ravages e.g. Old dynasties of ancient Egypt had records of severe epidemics and descriptions of disease from which they can be identified as well. In those days disease and pestilence was thought to be punishment rendered as a result of “bad deeds” and “evil thoughts”. Also, people knew that once he/she had been afflicted with disease and if survived, he/she would normally not contract it again. This phenomenon was described well by the historian Thucydides in his account of the plague of Athens of 430 B.C.
The first clear description (clinical) of smallpox was given by the tenth century Islamic physician Rhazes who differentiated it from measles and other disease for the first time. He also stated that recovery from small pox infection provides lasting immunity. Thus he gave the first explicit theory of acquired immunity.
Around 1000 A.D., ancient Chinese practiced a form of immunization by inhaling dried powders derived from the crusts of smallpox lesions. Around 15th century, a practice of applying powdered smallpox “rust” and inserting them with a pin or “poking” device into the skin was present. The process was referred as variolation.
During the 11th century, Avicenna hinted at another interesting theory of acquired immunity, which was explained on some years later by the Italian physician Girilamo Fracostaro in 1546 in his book on contagion. Fracostaro claimed that all disease was caused by small seeds or germs (seminaria) which might spread from person to person, each of which possesses a specific affinity for a given plant or animal, and for a given organ or humor. The ebullition is a kind of purifications of blood. Thus, he could explain satisfactorily all that was then known about smallpox. Fracostaro felt that his theory also implied simultaneously to other diseases as measles etc. This was criticized by Heironymus.
Some notable events in early immunology
• In 1718, Lady Mary wortley Montagu, the wife of the British ambassador Constantinople, observed the positive effects of variolation on the native population and had the technique performed on his own daughter.
• In 1774, Benjamin Jestley inoculated his wife with the vaccinia virus obtained from “farmer elfard of Chtenhall, near Yetminster. This is the first record of use of vaccinia virus to afford protection against smallpox.
• In 1798, Edward Jenner published his epoch-making report on a safer and more efficacious vaccine against smallpox, derived from cowpox pustules. (Latin: vacca-cow). Jenner
inoculated a young boy named James Phipps with material from cowpox lesions and then to prove infected the child with smallpox. As expected, the child did not develop smallpox. It took almost 100 years to apply this technique to other diseases.
Pasteur and Chicken cholera
Next major advance in immunology was the induction of immunity to cholera. Louis Pasteur isolated and grew bacteria causing cholera in pure culture. When chickens were inoculated they were found to develop cholera. After a summer vacation, he arranged for a public demonstration of the above set of experiment. During the demonstration, he accidentally inoculated the chickens with an old bacterial culture. This first led to illness but surprisingly chickens later recovered.
To again show that chickens inoculated with pure bacterial culture can develop cholera, Pasteur again performed the experiment. But this time he fell short of chickens hence , he used previously inoculated chickens. To his surprise the chickens survived and were totally protected from the disease. Pasteur then came to a conclusion “ that ageing weakens the virulence of the pathogen and these can be used for protection of the disease”. He called theses strains a vaccine.
He further said these attenuated bacteria still retained their capacity for stimulating the host to produce substances that protect against subsequent exposure to virulent organisms. This also explained the principle of Jenner’s successful use cowpox virus. Pasteur also applied this principle to the prevention of anthrax and it worked again. He produced the vaccine that was used for the famous demonstration at Povilly-le-fort in 1881.
• In 1885, Pasteur administered first vaccine to a boy named Joseph Meister who was bitten by a rabid dog.
• In 1886, Theobald Smith demonstrated that heat killed cultures of chicken cholera bacillus were also effective in protection from cholera. This demonstrated that the organism did not have to be living or viable ti induc the protection.
Pasteur was then given the challenge to make a vaccine for rabies. It has been established that thae rabies virus cannot be grown in lab cultures. It was given in rabbits by inoculating them with saliva from mad dogs. Then the brain and spinal cord were removed, dried, pulverized and mixed with glycerin. This, when injected into dogs, protected them against rabies. But for humans, it had to be different. Thus, another means of virulence attenuation was discovered which involved passage of the microorganism in an unnatural host.
• In 1901, Emil von Behring and Shiba Saburo Kitasato demonstrated that even the supernatants from the culture growth of diphtheria or tetanus organisms could confer immunity. They first suggested the mechanism to immunize.
Cellular Immunity
In 1883, Elie Metchnikoff was the first to give a cellular theory of immunity. He proposed that the phagocytic cell was the primary element in natural immunity. He also suggested that inflammatory response is an evolutionary mechanism designed to protect the organism.
Serotherapy
Emile Roux and Alexander Yersin in 1888 demonstrated that a soluble toxin could be isolated from the supernatants of the cultures of the diphtheria organism. It became evident that in some situations the exotoxin elaborated by the bacteria caused the disease.
Von Behring’s demonstration of antitoxin therapy and Paul Ehrlich’s findings about formation of neutralizing anti-toxin were other important events. Behring received the Nobel prize in physiological Medicine in 1901.
Cytotoxic Antibodies and Autoimmunity
In 1899, Jules Bordet demonstrated hemolysis in conjunction with the nonspecifically acting serum factor complement. Paul Ehrlich formulated his famous dictumof “horror autotoxicus”, which held that, for reasons unknown, an individual is unable to mount a destructive immune response against self-constituents.
In 1904, Karl Landsteiner and Donath reported the first observation of a true autoimmune disease paraoxysmal cold hemoglobinuria.
Serology and Immunodiagnosis
In 1896, phenomenon of bacterial agglutination was recognized which was used for identification and differentiation of bacteria using appropriate antisera, testing patient’s sera for previous exposure or immunity possessed.
Precipitin reaction when discovered was further advancement to include the assay of antigens and antibodies involving bacterial products and even non-bacterial agents.
G.H.F. Nuttall showed the application of immunology in the study of taxonomic relationship and even for forensic purposes by the reactions and cross-reaction of plant proteins or animal antisera.
Wassermann then developed a sero-diagnostic complement fixation test for Syphilis.
Allergy and Immunopathology
In 1902, Paul Portier and Charles Richet made discovery on anaphylaxis.. They demonstrated that even simple substances could, when injected into pre-sensitized individuals, cause severe systemic shock like symptoms and even death.
Shortly thereafter, Maurice Arthus demonstrated that simple antigens could cause local necrotizing lesions when they react with specific antibody in the skin of test animals, the so called arthus reaction. Soon it was demonstrated that hey fever and asthma belong to antibody mediated diseases.
Immunohematology
In series of studies starting in 1901, Karl Landsteiner showed that humans could be divided into several groups, depending on the presence, in their sera, of agglutinins specific for the RBCs of other humans. Later on, it was shown to be genetically determined by three allelic genes. The clinical application of this permitted to perform blood typing and modern blood transfusion techniques were developed.
Since that time, many other minor erythrocyte antigens have been identified and it has contributed significantly to forensic medicine, anthropological studies of racial relationships and mass migration.
Transplantation and Immunogenetics
By the end of 19th century, it was shown that tumors could be passed in experimental animals.
Usual graft rejection was being studied to use a treatment for human cancer.
In 1912, general rules of graft rejection were worked out and summarized in a book “ Heteroplastic and Homoplastic Transplantation”, by Goeorg Schle. He viewed rejection as an active response on the parts of the host immune system and coined the term “ transplantation immunity”.
In 1930s and 1940s, George Snell helped to define Major Histocompatible Complex (MHC) and gave an insight into graft rejection.
General Concepts of Immune System
Immunity (latin immunis, free of burden) is the study of an individual to resist infection.
Immunology is the study of immune system/ how the immune system resists the challenge to a foreign molecule.
All vertebrates have an immune system. Invertebrates have only primitive defense systems;
mostly they rely on phagocytic cells. Most of the information about immunity has come from the studies of responses of laboratory animals to injections of non infectious substances such as foreign proteins and polysaccharides. Any macromolecule, if it is foreign to the recipient, can induce an immune response, and is called an antigen.
The immune system is composed of many different cells, tissues and organs that recognize antigens and work to neutralize or destroy them. The immune responses are made only to molecules that are foreign to the host. This is an important property of immune system called self and non self recognition. Occasionally it fails to make this distinction and reacts against the self cells leading to autoimmune reactions.
There are two components of immunity namely Innate or non-specific immunity and adaptive or specific immunity. It is the specific immunity which has three important properties of diversity, specificity, memory and ability to differentiate self from non-self.
Early Theories of Immunity
Various theories have been given to understand the specificity of the antibodies i.e., response to millions of different antigens in a highly specific way.
• Selective theory was given by Paul Ehrlich in 1900. This theory stated that blood cells expressed a variety of side chain receptors capable of reacting and inactivating the foreign antigens. Their interaction results in more production and secretion of these side chain receptors with the same specificity. This theory thus states that receptor specificity is determined before their exposure to the antigen.
• The selective theory was challenged by Instructional hypothesis given in 1930 by Friedrich Brienl and Felix Haurowitz and later in 1940 by Linus Pauling. Instructional hypothesis states that antigen plays a central role in determining specificity of the antibody molecule.
This theory suggests that at antibodies are made as unfolded polypeptide chains whose final confirmation is determined by the antigen around which they become folded.
This theory however was abandoned when protein chemists discovered that the three- dimensional folded structure of a protein molecule is determined by its amino acid sequence.
• In 1950’s the idea of the selective theories was again used and Niels Jerne, David Talmadge and F. Macfarlane Burnet gave the clonal selection theory. This is supposed to be the most accurate and widely accepted theory.This theory proposes that during development, each lymphocyte becomes committed to react to a particular antigen before ever being exposed to it (Fig 1). The lymphocytes express membrane receptors specific for antigens. The binding of antigen to the receptors activates the cell, causing it to multiply into a clone of cells having same immunologic specificity. Thus a foreign antigen selectively stimulates those cells that bear complementary antigen specific receptors and are already committed to respond to it, leading to antigen specific immune responses. Thus, according to this theory, the immune system functions on the ‘ready made rather than the made to measure’ principle.
The cells responsible for specific immunity are lymphocytes (B and T lymphocytes), found in large numbers in the blood and lymph and in specialized lymphoid tissues as thymus, lymph nodes, spleen etc.
There are two classes of specific immune responses, mediated by two different classes of lymphocytes;
• Humoral immunity involving the production of antibody molecules, which neutralize the antigens, mediated by B lymphocytes.
• Cell mediated immunity involving the production of specialized cells that react mainly with foreign antigens on the surface of host cells, mediated by T lymphocytes.
The antigenic specificity of each lymphocyte is determined by antigen specific membrane receptors on their membrane. These receptors are attained by these cells during their maturation in primary lymphoid organs. B cells have membrane bound antibodies as their cell surface
receptors while T cells have a different kind of TCR, having antigen binding sites like antibodies on B cells.
Fig. 1: Clonal Selection theory
The immune response consists of three different phases:
a) All immune responses are initiated by the binding of foreign antigens to specific receptors on the mature lymphocytes, and this is the recognition phase.
b) Antigen recognition then leads to proliferation and differentiation of these cells into effector cells/ molecules; this is the activation phase.
c) These cells/molecules then engage themselves into a variety of immunological functions for elimination of the foreign antigen, which is the final stage of the immune response known as the effector phase.
The specific immune system has many important distinctive properties:
• Specificity i.e. the immune responses are specific to different antigens.
• Diversity - is because of the presence of a variety of membrane receptors on the surfaces of lymphocytes with specific antigen binding sites. Upon antigen exposure cells with specific receptors are recognized and activated as explained by the clonal selection theory. These different specific receptors makes them capable of responding to all antigens to which an individual is exposed to.
• Memory, is the property in which exposure of the immune system to a foreign antigen enhances its ability to respond again to that antigen.
When an individual is exposed to an antigen for the first time, it develops a primary immune response against the antigenic challenge (Fig 2). The primary immune response against an antigen is initiated as a lag period of several days, corresponding to 5-7 days, then it rises rapidly in an exponential manner and then gradually falls. Thus lymphocytes proliferate when stimulated by antigens leading to formation of effector cells/ effector molecules and also another population of cells called the memory cells. These cells survive for long periods and respond rapidly to the antigenic challenge.
If the individual is challenged again with the same antigen, a secondary immune response develops (Fig. 2), which is characterized by a shorter lag period (1-2 days), greater response for a longer duration. The secondary response reflects the activity of the memory cells.
Fig 2: Primary and Secondary response
• Discrimination of self and non self This is a property in which the immune system recognizes, responds and eliminates foreign antigens while not reacting to self cells. This state of immunologic unresponsiveness to self cells is called tolerance. Immune system is inherently capable of responding to both foreign and self antigens but ‘learns not to respond to self’ early in the development. It is achieved by elimination of lymphocytes that may express receptors for self antigens and also by functional inactivation of self- reactive lymphocytes after their encounter with self antigens.
Antigen Recognition by B and T Cells
B and T cells recognize specific regions of the antigens called the epitopes/antigenic determinants. Upon presence of an antigen B cells recognize antigen with the help of specific receptor (bound to the epitopes) and get activated. B cells recognize epitopes on the surfaces of bacteria, virus and those of soluble proteins, glycoproteins, polysaccharides or liopopolysaccharides. B cells can also function as Antigen presenting cells and help activation of TH cells. TH cells in turn secrete a variety of cytokines which stimulate various stages of B cell division and differentiation.
T cells on the other hand recognize antigens (or specifically bind to their respective epitopes) only when a degraded macromolecule is present along with Major Histocompatible Antigens, MHC molecule on the surface of a second cell called the Antigen presenting cell (APC). Hence mature T cells are referred to as MHC restricted. The antigen recognition by T cells via the TCR (T cell receptor) is thus aided by the MHC recognition of the antigen, which first binds to the antigen on sites other than the TCR binding sites. T cells also recognize protein epitopes displayed in association with MHC molecules on altered self cells ( virus infected/ cancerous cells).
MHC is a large genetic complex with multiple loci and encodes two sets of glycoproteins, class I and class II MHC molecules. Both these classes of molecules are highly polymorphic, with many different alleles within a species. Antigen presenting cells are responsible for internalizing the antigens and then degrading them into small peptides. MHC molecules on APC bind to antigenic peptides derived from the intracellular degradation of antigen molecules. The conversion of proteins into MHC associated peptide fragments on the APCs follows the processes of antigen processing and antigen presentation. Endogenous and exogenous antigens are processed by different methods.
Upon antigen activation, B cells take part in the humoral immunity, clonal proliferation of specific B cells occurs leading to the formation of antibody secreting plasma cells and memory B cells. While in the cell mediated branch T lymphocytes are involved and their proliferation leads to the formation of effector Th cells and cytotoxic T lymphocytes (CTLs) and also the memory T cells.
Th cell activation via the production of cytokines also regulates the proliferation of some of the non specific cells as NK cell and activated macrophages, which further aid in cell mediated immune response.
Thus, immune system is a multicomponent system wherein every component interact with each other and provide protection to the host from invading pathogens and altered self cells. On the other hand an improper function of the immune system can lead to any one of the following conditions:
• Hypersensitive reactions
• Autoimmune diseases
• Graft rejections
• Immunodeficiency
Cells and Organs of the Immune System
All blood cells arise from hematopoietic stem cell (HSC) in a process called hematopoiesis, formation of red and white blood cells. Stromal cells aid in the differentiation of HSC’c providing hematopoietic inducing microenvironment (HIM) made of a cellular matrix and growth factors in the form of cytokines as for e.g., multilineage colony stimulating factor (multi- CSF), macrophage colony stimulating factor (M-CSF), granulocyte macrophage colony stimulating factor (GM-CSF), Erythropoietin (EPO).
There are two pathways of development during hematopoiesis. In one the pluoripotent stem cell gives rise to common Lymphoid progenitor cell and in the other to a myeloid stem cell.
Lymphoid progenitor gives rise to Lymphoid cells while the myeloid stem cell gives rise to progenitors of RBC, WBC’s and platelets (Fig. 3). Unipotency of these cells is dependent on their particular growth factors and cytokines.
Fig. 3: Development of RBCs and WBCs during hematopoiesis. All lymphoid and myeloid progenitors develop from hematopoietic stem cells
Lymphoid Cells
Lymphocytes (Latin lympha, water and cyta, cell) are the cells of the specific immune system capable of specifically recognizing and distinguishing different antigens.
Lymphocytes are small round cells, only marginally bigger than the RBC’s and are filled with nucleus. They constitute 20-40 % of the total WBC’S and 99 % of the cells in the lymph. These cells circulate in blood and lymph and are capable of migrating into tissue spaces and lymphoid organs. On the basis of function and cell membrane components there are three subpopulations of the lymphocytes namely B cells, T cells and null cells.
Initially there are virgin/ naïve/ unprimed B or T cells that represent the resting cells (Fig. 4) not exposed to antigens. These are small (6 µm in dia), motile, non phagocytic with no morphologic distinction and a short life span. At this stage they can be marked with respect to Thy-1, a glycoprotein found on T but not on B cells. These small lymphocytes have a thin rim of cytoplasm with few mitochondria and poorly developed ER and Golgi apparatus. Nucleus is large, round and densely packed with chromatin.
After interaction with the antigen they enlarge into blast cells (15 µm) called lymphoblasts.
These have a higher cytoplasm: nucleus ratio and a more organellar complexity. Lymphoblasts proliferate and finally differentiate into effector cells or into memory cells. Effector cells help in elimination of the antigen and, have a short life span (few days –weeks).
Fig. 4: Different stages of B cells
B cells respond to soluble antigens and activated B cells develop into Ab secreting Plasma cells.
These cells have abundant ER and Golgi vesicles. These synthesize and secrete one of the five classes of Abs. They die in 1-2 weeks.
The effector cell population of the T cells are the cytokine secreting T –helper cell (Th cell) (Fig.
5) and the cytotoxic T lymphocyte (CTL). These contain very little endoplasmic reticulum and do not produce antibodies.
Memory cells provide life long immunity to many pathogens.
Fig 5: Development of T cells
B lymphocytes /B cells
Their name refers to bursa or bone marrow derived and are responsible for the humoral immunity. Stem cells in the bone marrow produce B cell precursors which mature into B cells in the bone marrow in mammals and in bursa of Fabricius in birds.
It is the only subset of lymphocyte capable of production of antibodies. Mature B cells contain cell surface, transmembrane antibody molecules associated with Ig-α and Ig-β heterodimer.
These complexes are called the B cell receptor (BCRs) which are diverse with respect to the Ag binding site. These receptors have IgM (in majority of cases) / IgD (Fig. 6) and serve as receptors for antigens.
Fig. 6: B Cell Receptor
Therefore, the total B cell population in an individual carries BCRs specific for many different antigens. Apart from BCRs there are many other membrane bound molecules on the mature B
cells, as for e.g., B cells have receptors for Fc part of some Immunoglobulin classes (Fc RII , CD 32 for IgG). They may also have receptors for complement (C3b) namely CD 35/CR1 and CD21/CR2.
They may express class II MHC molecules providing them their second immunologic role of functioning as APC. B7-1 (CD80) and B7-2 (CD86) are molecules on B cells which interact with CD 28 and CTLA-4 molecules on T cells, thus helping in co-stimulatory signals.
CD-40, binds to CD-40 ligand on TH cells. This interaction is critical for the development of Ab producing plasma B cells or memory B cells.
T Lymphocytes / T cells
These derive their name from their site of maturation in the Thymus. Their precursors arise in the bone marrow and then migrate into and mature in thymus. They can remain in thymus, circulate in blood or reside in lymphoid organs as lymph nodes and spleen. These are the cells responsible for cell mediated immune response and also for B cell activation.
Like B cells they also have Ag specific T cell receptors in combination with CD3 membrane molecule (TCR-CD3 complex) on their membrane surfaces (Fig. 7). TCRs have an Ag binding site but are structurally distinct from the Ab receptors on the B cells.
Fig. 7: T cell Receptor
Unlike B cells, T cells do not respond to soluble antigens. Instead they recognize processed antigenic fragments bound to MHC molecules exposed on the surface of APCs (signal 1), most of which are macrophages, Dendritic cells and B cells. Thus they are restricted to bind to antigen presented along with MHC on self cells. In addition for their activation T cells require another signal provided by co-stimulation (signal 2). Other membrane molecules expressed on T cell membranes include CD28, a receptor for B7 on B cells and CD45, a signal transduction molecule.
Types of T Cells
T cells are divided into regulatory T cells which regulate specific cells or the effector T cells which cause cytolysis and cell death of target cells. These different functional subpopulations are believed to express different membrane molecules.
Regulator T Cells control the development of effector T cells and also regulate activated B cell maintenance. Two subpopulations exist under this type namely T helper (Th) cells and T suppressor (Ts) cells
a) TH cells are known to express CD4 membrane glycoprotein and are restricted to recognize Ag bound to Class II MHC molecules. There are three subsets of Th cells namely Th1, Th2 and Th0.
(i) Th1 cells produce IL-2, IFN-γ , and TNF-β within a few hours of stimulation , which are involved in cellular immunity. This set of cytokines supports inflammation and is responsible for activation of certain T cells (for delayed type of hypersensitivity) and macrophages. IL-12 acts as their costimulator secreted by B cells or macrophages.
(ii) Th2 cells produce cytokines like IL-4, IL-5, IL-10, IL-3 several days after exposure to Ag. These are involved in humoral immunity as they act as helper for B cell activation and antibody mediated immune responses. A Th2 response is also associated with enhanced defense against helminthes. These cells have IL-1 receptors, which is secreted by macrophages and act as their costimulator.
(iii) Th0 cells are presumed to be undifferentiated precursors of Th1 and Th2 cells. It is more clearly existing in mice and is known to secrete lymphokines common to Th1 and Th2 subsets.
(b) T suppressor (Ts) cells
These cells have been postulated to suppress B cell and T cell responses. They are stimulated to proliferate by IL-2 produced from activated Th cells. It is a slow process and acts as a negative feedback control.
Effector T Cells
Another population of T cells are known to express CD8 membrane glycoprotein and are class I MHC restricted with respect to Ag stimulation and these are known as cytotoxic T cells (Tc).
Upon antigenic challenge they proliferate and differentiate into effector cells, the cytotoxic T lymphocytes (CTLs).
Apart from the two signals mentioned for T cell activation, theses cells also require a third signal in the form of cytokines (IL-2) secreted by Th cells . They secrete very few cytokines themselves and are capable of attacking virus infected cells, tumor cells and altered self cells.
Null Cells
It is a small population of lymphocytes that are neither T nor B cells and lack antigen binding receptors on their membranes. Most members of this population are large non phagocytic
lymphocytes with an extensive granular cytoplasm called NK cells. These form a distinct third population of lymphocytes.
They originate in bone marrow, their production is promoted by IL-2, IL-12 and IFN- . They represent 5-105 of the lymphocytes in human peripheral blood. They develop from a common precursor as that of T cells, but differentiate outside the thymus. These do not recirculate but are mainly found in secondary lymphoid organs.
NK cells do not express Igs, TCRs, CD4/CD8 molecules but they express 2 major surface molecules: CD56 (NKH-1) serves as an adhesion molecule, CD 11a/ CD18 (LFA1) . They also posses CD16, a membrane receptor for Fc part of the IgG which helps in their cytotoxic activity.
NK cells do not require prior sensitization and nor are they MHC restricted but are known to play important role against tumor cells and virus infected cells. They recognize their targets in one of the two ways: First they can function via a process called antibody dependent cell mediated cytotoxicity (ADCC) by using CD 16 receptor for Abs (Fig. 8). The second way NK cell recognize their target cells is with the aid of certain receptors (Killer activating receptors) which help them recognize abnormalities/ different molecules present on the surfaces of tumor/
virus infected cells. They can also analyze levels of MHC expression on cells. beige mice lack NK cells and are found to be more susceptible to tumor growth.
Fig. 8: Antibody dependent cell mediated cytotoxicity (ADCC)
Granulocytic Cells
Cells of the myeloid system are derived from the bone marrow (“myelos” is the greek for bone marrow) and these include granulocytes. These have irregular shaped nucleus and are called polymorphonuclear leukocytes (PMNs). Their cytoplasmic matrix has granules which contain reactive substances that kill microorganisms and enhance inflammation hence are also called inflammatory cells. On the basis of their cellular morphology and cytoplasmic staining characteristics they are classified as Neutrophils, Eosinophils or Basophils (Fig. 9).
Fig. 9: Cells of the Immune System
Neutrophils
(Latin neuter, neither; philien to love). These cells have a nucleus with three to five lobes (multilobed) and a granulated cytoplasm that stains with both acid and basic dyes. They constitute 50-70 % of the circulating WBCs and are produced by hematopoiesis in the bone marrow. These are the first cells to reach the site of inflammation.
These like macrophages have receptors for antibodies and complement proteins and are highly phagocytic. These use both the oxygen dependent and independent killing mechanisms for producing antimicrobial substances.
Eosinophils
(Greek eos, dawn and philien). These comprise 2-5 % of blood leukocytes in healthy non allergic individuals. They have bilobed nucleus connected by a slender thread of chromatin and the granules stain red with acidic dyes like eosin. Like neutrophils they are motile phagocytic cells that can migrate from the blood into tissue spaces.
Their role is important only in defense against protozoan and helminth parasites. They work mainly by releasing cationic proteins and reactive oxygen metabolites into the extracellular fluid to damage the parasite’s plasma membrane.
Eosinophils contain larger quantities of acid phosphatase and peroxidase. Eosinophil peroxidase is chemically distinct from that found in neutrophils, it is more efficient in killing certain organisms. In addition, each large eosinophil granule contains in its crystalline core a protein called major basic protein (MBP) which is highly toxic to invading parasitic worms.
Eosinophils also express receptors for IgE and are able to bind to IgE coated particles. These are also abundant at sites of immediate hypersensitivity (allergic) reactions, where they contribute to tissue injury and inflammation.
The growth and differentiation of eosinophils are stimulated by a helper T cell derived cytokine called IL-5 and T cell activation contributes to eosinophil accumulation at sites of parasitic infestation and allergic reactions.
Basophils
(Greek basis, base and philien). These cells have an irregular shaped nucleus with two lobes and the granules stain bluish black with basic dye methylene blue. They are the least numerous of the granulocytes and constitute about 0.5 % of blood leukocytes.
These are circulating cells whose functions are similar to those of tissue mast cells. Basophils are non phagocytic and function by releasing pharmacologically active substances such as histamine, serotonin and leukotriens from their cytoplasmic granules.
Basophils (like mast cells) possess high affinity receptors for IgE bind free IgE Abs and become coated with it. Subsequent interaction of antigens with bound IgE molecules serves as a strong stimulus for the release of granular contents (vasoactive mediators). These substances play a major role in certain allergic responses. Hence these are the effector cells of IgE mediated immediate type I hypersensitivity.
Mast Cells
These are the bone marrow derived cells formed by hematopoiesis which differentiate only when they leave the blood and enter the tissue. They are found at different locations including skin, connective tissue ( called connective tissue mast cells, CTMC) mucosal epithelial tissue of the respiratory, genitourinary and digestive tracts ( called mucosa associated mast cells, MMC) . CTMCs are T cell- independent while MMC appear to be dependent on T cell for proliferation.
They contain granules with histamine and other pharmacologically active substances. These along with basophils are important in the development of allergic responses. On the positive side they may play a role in immunity against parasites.
Dendritic Cells
These cells have many long filamentous cytoplasmic processes called dendrites (Fig.10). They have a long lobulated nuclei and a clear cytoplasm containing characteristic granules called Birbeck granules of unknown function.
Fig. 10: Dendritic cell
Dendritic cells are located throughout the body and in different locations they have different forms and functions.
Dendritic cells can recognize specific pathogen associated molecular patterns (PAMP) on microorganisms and play an important role in non-specific resistance. They can differentiate between potentially harmful microorganisms and self molecules. These cells are stimulated by endogenous activators as interferon-α, heat shock proteins and tumor necrosis factor that are released in response to microbial infection. They also play an important role in specific immune responses.
According to their location different names have been given to dendritic cells which process and present antigens to Th cells.
• Langerhans cells located in the skin and mucous membranes are capable of picking up antigens that enter via the skin and transport these antigens to draining lymph nodes, where immune response is initiated.
• Interestitial dendritic cells populate most of the organs e.g., heart, lungs, liver, kidney, gastrointestinal tract.
• Circulating dendritic cells are present in blood and those present in lymph are called the veiled cells.
There are two kinds of lymphoid dendritic cells:
• Interdigitating dendritic cells are present in T cell rich areas of secondary lymphoid organs i.e lymph nodes and spleen. They are also found in the thymic medulla.
All these dendritic cells constitutively express high levels of Class II MHC molecules and members of the co-stimulatory B7 family. Thus they are more potent APCs than macrophages and B cells , which function as APCs only after activation.
• A second class of lymphoid dendritic cell is known as follicular dendritic cells and are unrelated to interdigitating dendritic cells. They are present in the germinal centers of the lymphoid follicles in the lymph nodes, spleen and mucosal associated lymphoid tissues.
These do not express Class II MHC molecules, hence do not function as APCs. Instead they express high levels of membrane receptors for Ab and complement. With the help of these receptors these cells bind to the circulating Ag-Ab complexes and facilitate B cell activation.
Mononuclear Cells
The mononuclear phagocytic system consists of cells having a common lineage whose primary function is phagocytosis. These cells were identified by the German scientist Ludwig Aschoff in the early 20th century as the cells which took up dyes injected intravenously. There are two types of mononuclear cells namely monocytes and macrophages. During hematopoiesis in the bone marrow, granulocyte-monocyte progenitor cells differentiate into promonocytes, which
leave the bone marrow and enter the blood, where they further differentiate into mature mononocytes.
Monocytes (Greek monos, single and cyte, cell) are mononuclear phagocytic leukocytes, 10- 15µm in diameter, with an ovoid or kidney shaped nucleus and granules in the cytoplasm that stain grey-blue. They circulate in the blood for 8 hours, enlarge, migrate to the tissues and mature into macrophages.
Macrophages (Greek macros, large and phagein, to eat) are 5-10 times larger than monocytes, show an increase in number and complexity of intracellular organelles (especially lysosomes), have increased phagocytic capability, produce higher levels of hydrolytic enzymes and produce a number of soluble factors. Their plasma membrane is covered with microvilli. Macrophages also have receptors for antibodies and complement.
These spread throughout the body, some take up residence in particular tissues and are called fixed macrophages while others remain motile and are called free or wandering macrophages.
Fixed macrophages according to their tissue location are given some special names like:
• Alveolar macrophages in the lung;
• histiocytes in connective tissues;
• kupffer cells in the liver;
• mesangial cells in the kidney;
• microglial cells in the brain and
• osteoclasts in bone.
Some mononuclear phagocytes may differentiate into another cell type called the dendritic cell.
Macrophages are normally in the resting state and are activated by many stimuli (the initial stimuli provided by phagocytosis of particulate antigens, other stimuli include cytokine( γ-IFN) production by activated Th cells, mediators of inflammatory response and components of bacterial cell walls) and these cells are important in non specific resistance.
Activated macrophages show increased level of lysosomal enzymes, more phagocytic activity, higher potential to kill ingested microbes, increased expression of antibody, complement and transferring receptors, increased secretion of inflammatory mediators and increased ability to activate T cells.
They also secrete many cytotoxic proteins against pathogens, virus infected cells, tumor cells and intracellular bacteria. They express higher levels of Class II MHC molecules and function as more potent APCs. Thus these cells are important in the bidirectional interactions between innate and specific immunity.
Macrophage functions
Macrophages like neutrophils are capable of phagocytosis. They are attracted by substances like cytokines, products of immune responses as anaphylatoxins (C5a), bacterial products as N- formyl methionyl peptides, factors released by damaged cells and extracellular matrix like fragments of collagen, elastin (a fibrinogen generated by dying neutrophils) and move by
chemotaxis. They are capable of phagocytosing microbes (bacterial cells or viral particles), macromolecules and even self tissues (injured/ dead tissue).
The process involves the formation of pseudopodia, membrane extensions around the adhered material, formation of phagosome, phagolysosome, digestion of the ingested material and their exocytosis. Since macrophages have receptors for certain classes of antibodies and the complement proteins which can bind to antigen they are involved in increased phagocytosis by opsonization. Thus, antibodies and complement act as opsonins, molecules that can bind to antigen as well as macrophages and enhance phagocytosis.
There are two methods of killing employed by macrophages namely O2- dependent killing and O2- independent killing mechanisms:
• Oxygen- dependent killing involves the production of a number of reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates. It involves a metabolic process called respiratory burst resulting in the activation of a membrane bound oxidase which catalyzes the reduction of oxygen to superoxide anion which can also generate hydroxyl radicals and hydrogen peroxide (H2O2) and all of these are powerful oxidizing agents. Action of myeloperoxidase in phagolysosomes produces hypochlorite (toxic to microbes) from water and chloride ions.
Macrophages also express high levels of Nitric oxide synthetase (NOS) when activated with LPS/ muramyl dipeptide and bacterial cell wall components. NOS is responsible for production of nitric oxide (an antimicrobial agent) from oxidation of L-arginine.
• Oxygen-independent killing involves lysozyme and other hydrolytic enzymes which can function in the absence of oxygen. Activated macrophages are also capable of producing defensins, antimicrobial and cytotoxic peptides. These are capable of forming ion permeable channels in bacterial cell membranes. Activated macrophages can also produce TNF-α , cytotoxic to tumor cells.
Organs of the Immune System
Most of the lymphocytes, mononuclear phagocytes and other accessory cells are concentrated in defined tissues or organs which are also the sites for transport and concentration of foreign antigens. These tissues/ organs are morphologically and functionally diverse and aid in the development of immune responses. Many lymphocytes recirculate and constantly exchange between the circulation and tissues via lymphatic system, and secondary lymphoid organs (Fig.11).
These are classified broadly into:
• Primary Lymphoid Organs also known as central or generative organs where maturation of lymphocytes occur and they become committed to a particular antigenic specificity. This includes Bone marrow where all lymphocytes arise and Thymus where T cells mature.
• The Secondary Lymphoid Organs or peripheral organs are the site for interaction of mature lymphocytes with the antigen and development of the immune response. These include lymph nodes, spleen and various mucosal associated lymphoid tissues or MALT.
In addition, there are tertiary lymphoid tissues, represented by cutaneous associated lymphoid tissues (CALT).
Fig. 11: Different lymphoid tissues in the human body
Primary Lymphoid Organs Bone Marrow
Bone marrow is the site for differentiation of Hematopoietic stem cells into specific cells lineages. In addition maturation of B cells also occurs in Bone marrow. It consists of a
meshwork of reticular tissue divided into two distinct compartments, a hematopoietic compartment and a vascular compartment (Fig.12).
The hematopoietic areas contain precursors of all blood cells, and are enclosed by a layer of reticular cells. Lymphocytes are either scattered among all cells or are clustered in discrete groups. The vascular compartment of the bone marrow consists of blood sinuses lined by endothelial cells and crossed by reticular cells and macrophages. This area is also meant for entrapping bacteria. The stromal cells within the bone marrow interact directly with the B cells and secrete various cytokines that are required for the development of selection processes within the bone marrow help to eliminate B cells with self reactive antibody receptors.
Fig. 12: A section of Bone Marrow
In Birds, the primary site for B cell maturation is a lymphoid organ called the Bursa of Fabricus, a gut associated lymphoid tissue.
Thymus
The thymus is a flat bilobed organ situated above the heart. Each lobe is surrounded by a connective tissue capsule and is divided into lobules by fibrous septa (Fig. 13). Each lobule consists of an outer cortex which contains a dense population of immature T cells called Thymocytes and an inner lighter staining medulla sparsely populated with thymocytes and a network of stromal cells along with interdigitating dendritic cells and macrophages. Non lymphoid epithelial cells are present throughout the thymus and help in the growth and maturation of Thymocytes. Also present within the medulla are round bodies called Hassall’s corpuscles, composed of tightly packed whorls of epithelial cells, these cells are called Nurse cells, Nurse cells are thyminc epithelial cells in the outer cortex and have long membrane extensions that can surround up to 50 thymocytes. The thymus has a rich vascular supply and efferent lymphatic vessels that drain into lymph nodes. The functions of thymus can be studied by thymectomy (removing the thymus and monitoring the consequences. In humans in a condition called Di George’s syndrome, wherein thymus fails to develop there is an increase in
infections. However in adults surgical removal of the thymus produces no immediately obvious result as ageing shows a decline in thymic function. This decline may play some role in the decline in immune function during ageing in humans and mice.
Fig. 13: A section of Thymus
The thymic epithelial cells release several hormones, which are all polypeptides ranging from 1 to 15KDa and include thymosine, thymoproteins, thymic hormonal factors, thymulin and thymostimulins. These also produce interleukin-1. At the juncture of the cortex and medulla are present interdigitating dendritic cells which interact with developing thymocytes.
Thymic Maturation and Selection of T-Cells
Thymus is the site of maturation of lymphocytes. The progenitor T cells produced by hematopoeisis enter the thymus and multiply within the cortex. These then migrate towards cortex and undergo maturation and finally leave the thymus through post capillary venules. T cells during maturation within the thymus attain antigenic diversity of their T cell receptors by a series of gene rearrangements. Then they are subjected to a two step selection process by which T cells become MHC restricted (positive selection) and self tolerant (negative selection) respectively (Fig.14). These immunocompetent T cells exit the thymus and enter the blood and peripheral lymphoid tissues. The selection process within the thymus are assisted by thymic stromal cells, which express high levels of class I and class II MHC molecules.
Fig. 14: Thymic selection of T cell
Secondary Lymphoid Organs
The secondary lymphoid organs consist of mature T and B lymphocytes for action against antigens. Many such tissue organizations are present in the body. Many such tissues are present along the vesicles of the lymphatic system.
(i) One such tissue consist of diffuse collections of lymphocytes and are present in the lung and the lamina propria of the intestine.
(ii) A higher degree of lymphoid tissue is organized into lymphoid follicles. These follicles comprise of aggregates of lymphoid and non-lymphoid cells surrounded by lymphatic capillaries. Before activation these follicles are called primary follicles. These consist of a network of follicular dendritic cells and resting B cells. Upon antigen exposure the primary follicle is converted into a secondary follicle. This is formed of a ring of B lymphocytes surrounding a centre called the germinal centre which is the site of proliferating B lymphocytes. Also present in these follicles are non-dividing B cells, Th cells and macrophages.
(iii) Most highly organized secondary lymphoid organs are lymph nodes and the spleen.
Lymph Nodes
Lymph nodes are encapsulated round or bean shaped structures that lie at the junction of lymphatic vessels they are the first organized lymphoid structure meant to encounter antigens entering the tissue spaces. These consist of a fibrous reticular network filled with lymphocytes, macrophages and dendritic cell, which help in trapping any particulate antigen that is brought in the node along with the lymph. Lymphatic sinuses penetrate the node. A subcapsular sinus is located immediately under the connective tissue capsule and the other sinuses pass through the node but are most prominent in the medulla. Lymph percolate in the node through numerous afferent lymphatic vessels which empty lymph into subcapsular tissue and moves to afferent lymphatic vessels. Internally the lymph node is divided into an outer cortex middle paracortex and a central medulla (Fig.15). The cortical region predominantly contains B cells alongwith macrophages and follicular dendritic cells are arranged as primary follicles. After antigenic
challenge primary follicle expands to form secondary follicle containing a central germinal centre. Germinal centres are the site of remarkable B cell proliferation, here the B cells undergo the process of somatic mutation (wherein the cells ability to bind antigen changes at random and selection of B cells producing high affinity antibodies occurs). These are also the site of immunoglobulin class switching and memory cell formation. Follicular dendritic cells here help display antigens on their surfaces for B cell activation. Antibodies producing plasma cells develop outside the germinal centre and may migrate out of lymph nodes to other tissues.
The paracortical region beneath the cortex is mainly populated by T lymphocytes. Most of these are CD4+ T cells intermingled with few of CD8+ T cells. This region also contains interdigitating dendritic cells expressing high levels of class II MHC molecules, thereby helping in T cell activation.
Fig. 15: A section of Lymph Node
Based on the cellular content, the cortex is referred as a thymus -independent area and paracortex is termed as thymus -dependent area. The central region of the lymph node, the medulla has a few lymphoid lineage cells, many of which are antibody secreting plasma cells and large numbers of macrophages and dendritic cells.
Antigen enters the node by the lymph, it is trapped processed and presented to T cells by the help of interdigiting dendritic cells in the paracortex. Initial B cell activation also occurs within the T- cell rich paracortical region. Activated TH cells and B cells then form small foci at the edges of paracortex. A few B cells and TH cells migrate to primary follicles and initiate their conversion into secondary follicle. Some of the plasma cells thus produced move to the medulla and others migrate to the bone marrow. Thus after antigen exposure the lymph coming out of the node has high quantities of antibodies and a higher concentration of the lymphocytes. The increase of lymphocytes is because of proliferation and also due to migration of blood borne lymphocytes into the node which is assisted by passing between specialized endothelial cells that line the post capillary venules of the node. These migrating T cells thus percolate through the
lymph node and then re-enter the lymphatic circulation via the efferent lymph vessels. Thus antigen challenge of a lymph node measures the migration of these cells, leading to an increased count of lymphocytes in the node which leads to visible swelling of the node.
Spleen
The spleen is a large ovoid secondary lymphoid organ situated high in the left abdominal cavity.
Spleen is the major site of immune responses to blood borne antigens and thus responds to systemic infections. The spleen is not supplied by lymphatic vessels but is supplied by a splenic artery.
Outermost covering of the spleen is a capsule which sends internally a number of projections known as trabeculae thereby dividing it into two compartments, the red pulp and the white pulp (Fig. 16) which are separated by a diffuse marginal zone. The spleen act as an important filter for the blood, because of the red pulp region. This consists of a network of sinusoids populated by macrophages and numerous red blood cells. These red pulp macrophages clear the blood of unwanted foreign substances, and cause destruction and removal of old and defective red blood cells.
Fig. 16: A section of Spleen
The white pulp surrounds the branches of the splenic artery, forming the periarteriolar lymphoid sheath (PALS) mainly containing T lymphocytes. The marginal zone on the outer side of PALS is rich in B cells organized into primary follicles, alongwith macrophaged and CD4+ T cells.
Blood borne antigens and lymphocytes enter the spleen through the splenic artery, which empties into the marginal zone. Here the antigens are trapped by interdigitating dendritic cells and the initial activation of B and T cells occur in T cell rich PALS. Activated B cells together with some Th cells move to primary follicles in the marginal zone. These in the presence of antigen then expand into secondary follicles.
Mucosal associated Lymphoid Tissues
In addition to highly organized secondary lymphoid structures as lymph nodes and spleen, lymphocytes are also found either scattered or in aggregates in many tissues such as the lamina propria of intestinal villi. A good example of such collections include the group of organized lymphoid tissues in association with mucous membranes lining the digestive, respiratory and urinogenital systems and are collectively called mucosal associated lymphoid tissue (MALT).
Some of these are anatomically well organized and have unique properties a for example tonsils, appendix and the Peyer’s patches and these form the gut associated lymphoid tissue (GALT) (Fig.17).
Fig. 17: A section of MALT
Mucous membrane lining the gastrointestinal tract has lymphoid cells found at varied locations.
The outer mucosal epithelial layers contain intra epithelial lymphocytes (IELS). Under the epithelial layer is present the lamina propria and it contains a large number of B cells, plasma cells, activated TH cells and macrophages in loose clusters. The submucosal layer beneath the lamina propria contains Peyer’s patches, which are nodules of 30-40 lymphoid follicles . the epithelial cells of mucus membranes have specialized cells called M cells, capable of antigen transport and help in promoting the immune response by the mucosal associated lymphoid tissue.
Over the organized lymphoid follicles, M cells can take an antigen from the lumen of digestive, respiratory and urinogenital tracts by endocytosis. The antigen is transported across the cell and released into the large basolateral pocket. This transported antigen then activates B cells in the underlying lymphoid follicles which differentiate into IgA producing plasma cells. Antibodies are then released in the lumen where they can interact with the antigens.
Cutaneous associated Lymphoid Tissue
Skin is an important anatomic barrier to the external environment and is important in non specific immune responses. In addition, epidermis, the outer layer of the skin contains specialized cells called the Langerhan’s cells, a type of dendritic cell which forms the skin associated lymphoid tissue (SALT). Here Langerhan’s cells can internalize antigen and then migrate to the lymph nodes where they differentiate into interdigitating dendritic cells. These cells express high levels of class II MHC molecules and function as activators of naïve Th cells.
The epidermis also contains intra-epidermal lymphocytes, to combat antigens entering through the skin.
Innate and Acquired Immunity
Two functional divisions of the immune system are non-specific immunity called the innate immunity and the specific component, the acquired immunity.
The innate immunity acts as a first line of defense against infectious agents. It includes resistance mechanisms that are not specific to a particular pathogen. It functions similarly against most infectious agents and helps in checking the potential pathogens before they establish an infection.
If these defenses are breached, the acquired immunity plays its role which is specific for each antigen. Both of these immune components function in a cooperative manner.
Innate Immunity
This is also called the non- specific resistance/ natural/ native immunity and mounts its effect immediately after the exposure to an antigen. This immunity consists of soluble factors as lysozyme, complement proteins, acute phase proteins, interferons while its cellular components include phagocytic cells and NK cells (Fig.18). There are many different levels of innate immunity
Physical and Anatomic Barriers
These are meant for preventing the entry o the pathogens. Included under this category are the skin and the mucous membranes.
Skin
The intact skin forms a very effective mechanical barrier to microbial invasion and contributes highly to the non- specific resistance. The reasons for this are :
Most infectious agents cannot penetrate the intact skin because its outer layer consists of thick, closely packed cells called keratinocytes which produce keratins. Keratins are the scleroproteins
present in hair, nails and outer skin cells that microorganisms cannot attack enzymatically.
Keratinocytes also secrete many proteins responsible for inflammation.
Fig. 18: Innate Immunity and adaptive immunity
Microorganisms that adhere on the outer epithelial cells are removed by the continuous shedding of these cells and normal washings by the individuals.
The dry layer of skin does not support rapid growth of microbes. The sebaceous glands in the dermis of skin produce sebum, an oily secretion containing fatty acids and lactic acids and help maintain the pH of the skin towards acidity thereby inhibiting the growth of many microorganisms. Sebum thus forms a protective film over the skin.
The normal microbiota occupies the attachment sites and competes for the nutrients. Hence it doesn’t allow other pathogens to infect.
Inspite of all these defenses if some pathogenic organisms gain access to the tissue under the skin, they are encountered by SALT. These function to prevent their entry into the blood by confining them to the entry point only. One type of SALT cell is the Langerhans cell (dendritic cell) which phagocytose the antigens, migrate to the nearby Lymph nodes and differentiate into interdigitating dendritic cell. Thus they activate nearby T cells.
Another type of SALT cell in the epidermis is intraepidermal lymphocyte.
Also present in the dermal layer of skin are many tissue macrophages and can help in phagocytosis.
• Mucous membranes lining the eye (conjunctiva), respiratory, digestive and urogenital systems avoid microbial invasions because they have intact squamous epithelium and mucous secretions. These resist microbial invasion and trap the microbes.
• Mucosal surfaces are also known to be bathed in specific antimicrobial secretions.
Examples include Lysozyme (muramidase), an enzyme that lyses the bacterial cells;
Lactoperoxidase, an enzyme that produces superoxide radicals which is toxic to many microorganisms; many proteins which prevent attachment of microbes and iron- binding Lactoferin, released by activated macrophages and PMNs, which can limit the multiplication of the microbes.
These like skin have immune barrier in the form of MALT (GALT and BALT). MALT system makes use of M cells.
• In the respiratory system, the mucociliary blanket traps mucosal deposited microorganisms and their ciliary action transports them away from the lungs. Coughing and sneezing also help clear the respiratory system of the microbes. Salivation also washes microbes from the mouth and nasopharyngeal areas into the stomach. If the microbes still reach the alveoli of the lungs they are killed by the alveolar macrophages.
• Within the gastrointestinal tract, they are killed by the gastric juice (a mixture of hydrochloric acid, proteolytic enzymes and mucus; having pH 2-3) in the stomach.
Protozoan cysts, Clostridium and Staphylococcus toxins however are not destroyed by the gastric juice. Many organisms can still reach the small intestine along with the food particles.
• In the small intestine they are attacked by the pancreatic enzymes, bile, enzymes in the intestinal secretions and the GALT system. Peristalsis and normal loss of the columnar epithelial cells help remove the microorganisms. In addition the normal commensals in the intestine help prevent the establishment of the pathogenic organisms.
• The mucous layers of the intestinal tracts are known to contain Paneth cells which produce lysozyme and peptides called cryptins which are toxic for some bacteria.
Normally the kidneys, ureters and the urinary bladders of mammals are sterile because of many such factors.
Physiologic and Chemical Barriers
This set of check includes factors like temperature, pH and various soluble factors.
• The body temperature if doesn’t support the growth of pathogenic organism the individual will not be infected.
• pH effect can be seen as in the case of gastric juice.
• The soluble factors include components like Lysozyme, interferons; proteins produced by virus infected cells and generally induces an antiviral state, complement proteins; a group of serum proteins whose function is to damage the membrane of the microbes.
• Blood, Lymph and other body fluids also contain some other defensive chemicals, such as
o Bacteriocins produced by the normal microbiota. These are toxic proteins (eg.
Colicin) lethal to related species. These bind to specific receptors on the cell envelope of sensitive target bacteria and cause cell lysis. They can also attack specific intracellular sites as ribosomes or disrupt energy production.
o Beta lysin, a cationic polypeptide released from the blood platelets, known to kill some Gram positive bacteria by disrupting their plasma membrane.
o Leukins, plakins, cecropins and phagocytin are some other cationic polypeptides.
Phagocytic Cells
Third defense system of the innate immunity is the presence of phagocytic cells. Macrophages and the neutrophils in circulation are capable of phagocytoses and removal of foreign substances and even the whole pathogenic organisms.
Inflammation
(Latin, inflammation, to set on fire) is an important non- specific defense reaction to injury. Thus the body’s response to a tissue injury/ irritation is called an inflammatory response. This involves the necessary movement of immune system components (like complement and phagocytic cells) to the site of infection.
Acute inflammation is the immediate response to injury or cell death. The cardinal signs of inflammation include redness (rubor), warmth (calor), pain (dolor), swelling (tumor) and altered function (functio laesa).
Acute inflammatory response begins when injured tissue cells release chemical signals that activate the endothelium of the capillaries.
Three major events occur during this response (Fig.19).
• An increased blood supply to the infected area characterized by vasodilation which is an increase in the diameter of the blood vessels, It also involves engorgement of blood capillaries causing tissue redness and an increase in the temperature.
• An increase in the capillary permeability because of contraction of endothelium which pulls apart the endothelial cells leading to escape of fluid through intercellular spaces, thereby causing edema.
• Increase in the capillary permeability facilitates migration of leukocytes, particularly the neutrophils out of the capillaries into the surrounding tissue.
This emigration of phagocytic cells includes:
• Adherence of the cells to the endothelial wall of the blood vessels in a process called margination.
• This is followed by diapedesis / extravasation which is the movement of these cells out into the tissues.
• The last step is their migration towards the site of infection by a process known as chemotaxis.
Fig. 19: Reactions of Inflammation
Phagocytic cells begin to phagocytose pathogenic organism, they release lytic enzymes, some of the healthy nearby cells are also damaged in the process. The last stage is the pus formation which induces accumulation of dead cells, digested material and fluid.
Several chemical mediators take part in the inflammatory response which may be secreted by the pathogen itself, damaged cells, by plasma enzyme systems and some are the products of various white blood cells.
These chemical mediators include serum protein called acute phase proteins. They help in complement mediated lysis or increased phagocytosis.
Another example includes histamine which causes vasodilation and increase in capillary permeability; Kinins activated by tissue injury also help in vasodilation and increase in capillary permeability but they are also responsible for stimulation of pain receptors.
Blood clotting enzyme system further prevents the spread of the infection. Ultimately the response ends with tissue repair and regeneration.
Innate immunity basically comes from the general structure and function of an individual hence it lacks immunological memory i.e., it occurs to the same extent each time the individual encounters foreign antigens. This resistance is not improved by repeated infection.
Acquired Immunity
This is a specific immune response to resist a foreign agent. Hence is also called specific/
adaptive immunity. The antigens in this immune response cause specific cells to be activated and different ways and molecules are used to eliminate distinct molecules. Moreover these responses improve on repeated exposure to the foreign agents. Also this response has an ability to remember and respond more vigorously to repeated exposures to the same foreign antigens. This is characterized by four important characteristics:
• Antigenic specificity which allows it to distinguish between different antigens.
• Diversity by which the system is capable of producing a variety of antigenic receptors and can recognize billions of different antigens.
• Immunological memory by which the specific immune response remembers its exposure to an antigen. So subsequent encounters with the same antigen induces a heightened immune response. This enables it to confer life long immunity to many infectious agents after an initial exposure.
• Self and non self recognition which confers them with the property of not responding to self molecules and thus prevent autoimmunity/ an inappropriate immune response.
Cells involved in acquired immunity are lymphocytes and the soluble factors are the antibodies produced by the activated B cells or the CTLs. The specific and non-specific immune responses usually work together to eliminate pathogenic organisms.
The phagocytic cells are known to activate specific immune response. The soluble factors of the specific immune response augment the activity of the phagocytic cells. Also the intensity of the inflammatory response is again regulated by the factors of the specific immune response.
Types of Acquired Immunity
Thus set of immunity can be achieved by natural or artificial means and actively/ passively.
Naturally Acquired Immunity This is of two types:
Naturally acquired active immunity
This occurs when an individual has infection by a foreign antigen and responds by producing antibodies and activated lymphocytes for evading out the antigen. This kind of immunity can be either lifelong, as with measles or chicken pox or only for a few years as with tetanus.
Naturally acquired passive immunity
This involves the transfer of antibodies from one host to another. This type of immunity is seen when maternal antibodies cross the placenta and provide immunity to the developing foetus against diseases like diphtheria/ polio. Certain antibodies can also pass from the female to her
