Immunoglobulins are glycoproteins (that is, they contain sugars as well as amino acids), which bind specifically to the antigen that provoked their formation. As mentioned previously, when the two molecules are bound together, this combination of antibody bound to antigen is known as an immune complex (Figure 5.5).
All antibodies have a four-chain structure and are made up of two identical light chains and two identical heavy chains (Figure 5.6).
The chains are made out of polypeptides that are bound to each other by disulphide bonds. The light chains are identified by the Greek letters lambda (l) and kappa (k). There are five types of heavy chains and they are also identified by Greek letters: gamma (g), alpha (a), mu (m), delta (d) and epsilon (e). The heavy chains are found, respectively, in classes of immunoglobulin known as IgG, IgA, IgM, IgD and IgE. The kappa and lambda short chains are found in all five classes.
Some parts of both light chains and heavy chains are made up of amino acid combinations that never vary (the constant region). However, the amino acid sequences in other parts of both light and heavy chains in the antibody constantly change, that is, the amino acid sequence differs from molecule to molecule. This part of the antibody fragment is known as the hypervariable region.
In the laboratory, antibody molecules can be split into three large pieces by a protein-digesting enzyme (papain). Two of these pieces are exactly the same and contain the antibody binding site. They are known as the Fab fragments. These fragments consist of the entire light chain and almost half of the heavy chain (linked together by the disulphide
Stem cell B-cell
precursor
Bone marrow
Mature B-lymphocyte
B Plasma
cell
FIGURE 5.4 The maturation of a B-lymphocyte.
bonds). The hypervariable region of the antibody is in the Fab fragment and this is where the specific lock is made to bind onto a specific antigen. The locks (that is, the complete antibody repertoire) in the hypervariable region are made as a result of constant antibody gene re-arrangement during the growth and development of B-lymphocytes, long before exposure to antigens with the corresponding epitopes. This results in a vast number (thousands) of combinations of molecules that will recognize similar combinations of amino acid molecules on the surface of the thousands of different antigens they may meet throughout the course of our life.
The remaining fragment of the antibody is known as the Fc fragment and is important in determining the essential characteristics of the antibody, for example whether or not it can cross the placenta and enter the fetal circulation. The Fc fragment also binds to certain antigens, such as bacteria, and activates the complement system. In addition, by Fc binding, the antigen becomes ‘buttered’ (as in G.B. Shaw’s observation, discussed previously), the Fc fragments of certain types of immunoglobulins acting as opsonins, sticking to the antigen and attracting phagocytes.
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Plasma cell
Antibodies
Immune complex Antigen
FIGURE 5.5 Immune complex.
Antibodies are very diverse and provide different and specific binding sites to the thousands of different antigenic shapes that they can potentially encounter throughout life.
They protect the individual in several ways. First, the antibody attacks the cell wall of the invading pathogen, weakening and eventually destroying it; this is known as lysis. Second, and more importantly, when the antibody ‘coats’ or attaches itself to an antigen, the formation of immune complexes activates complement. As discussed previously, complement has two important functions: it initiates a local inflammatory reaction at the invasion site, which increases the local blood supply, and inflammatory mediators, including chemokines and other cytokines (discussed earlier), chemically attract neutrophils, monocytes and macrophages to the area under attack. This chemical attraction of phagocytic cells to the invasion site is known as chemotaxis. Some microorganisms are destroyed by direct cell wall damage when coated with complement.
We can see that it is critically important for plasma cells to secrete additional circulating antibody. As described above, immunoglobulins are classified according to the structure of their heavy chains into IgG, IgM, IgA, IgD and IgE. Immunoglobulins in the IgG and IgA classes are also further divided into subclasses, for example IgG1 and IgG2. The following are a few points about the different classes of immunoglobulins.
Disulphide bonds
Variable region
Constant region
Light chain
Heavy chain
Fab portion
Fab portion
Fc portion
FIGURE 5.6 Structure of an antibody.
IgM
This is a large immunoglobulin (macroglobulin), which appears early in the course of an infectious disease and disappears as the patient recovers. Consequently, its presence in the blood is a marker of acute infection. Because this antibody appears early in the infection, IgM is mainly confined to the bloodstream and accounts for about 10 per cent of the total amount of circulating immunoglobulins. As IgM is highly efficient at binding and agglutinating microorganisms, it plays an important role as the first line of defence in bacteraemia (bacterial infection of the blood). Because it can easily cross the placenta from mother to fetus, elevated levels in the neonate indicate intrauterine infection.
IgG (gamma-globulin)
This small antibody accounts for 70 per cent of all the immunoglobulin formed and appears after IgM, during a first or primary response to an antigen. Memory cells for IgG are produced during a primary response, and therefore larger quantities of IgG are produced on further infection (secondary response), appearing earlier in the infection than the IgG produced during a primary response. By contrast, IgM is not associated with memory and the amounts produced and the time scales in primary and secondary antibody responses are very similar. There are four subclasses of this immunoglobulin. Because of its small size, IgG diffuses more easily than other antibodies out of the bloodstream and into the tissue fluids of the body, where it is the principal antibody responsible for neutralizing bacterial toxins, binding to microorganisms, activating complement and promoting phagocytosis. It also readily crosses the placenta, and maternal IgG provides the principal means of defence against infection for the first few months of a baby’s life. Because it appears after IgM and memory cells remain in the body for long periods of time (years or a lifetime), its presence may indicate previous exposure, vaccination or acute infection.
IgA
This immunoglobulin accounts for up to 13 per cent of the total amount of immunoglobulin formed, but it accounts for more than 90 per cent of the immunoglobulin found on mucosal surfacessuch as the nose and mouth, and protects the body by blocking and neutralizing antigens that enter by these routes. IgA is found in high levels in seromucous secretions, for example in tears, sweat, saliva, colostrum, urine, and respiratory, gastrointestinal and genito-urinary secretions. This immunoglobulin does not cross the placenta. However, it is present in large quantities in colostrum and breast milk, protecting the infant from infection via breastfeeding. There are two subclasses of this immunoglobulin.
IgD
This class of antibody accounts for less than 1 per cent of the total amount of circulating immunoglobulins and not much is known for certain about its specific function. However, it is found residing on the surface of B-lymphocytes, along with IgM, and they both probably operate as mutually interacting antigen receptors, controlling further lymphocyte activation and suppression.2
IgE
This antibody accounts for only about 0.002 per cent of immunoglobulin formed. It binds to the surface membranes of mast cells and basophils, causing these cells to release vasoactive substances such as histamine, heparin and serotonin, which are responsible for the common
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signs and symptoms of acute allergic reactions. Mast cells are fixed in tissues such as the lungs, whereas basophils are circulating mast cells. As elevated levels of IgE are found during parasitic infections, IgE may be important in protecting against this type of infection.