6- Antigenicity
Antigens are distinguishing features that our immune systems use to
recognize and remember invaders. Microbes can evade our immune defenses by changing their antigens. In this lesson, learn about antigenic shift with influenza virus as an example.
Definitions:
Antigens are the specific molecular structures that antibodies and other receptors in our immune systems recognize. Antigens and antibodies also form the basis of immunological memory, which is when the immune system responds faster and more powerfully to an infection that it has seen before.
From a microbe's point of view, it's much easier to infect a host, survive and replicate if the host's immune system doesn't recognize you. An antigenic shift is a sudden, drastic change in a microbe's antigens, and it can help microbes evade the immune system. Unfortunately for the host, this often makes microbes more pathogenic, or capable of causing disease.
It's important not to confuse antigenic shift with antigenic drift, which is a gradual and more minor change in a microbe's antigens over time.
Acute viral infections can be rapidly cleared by the immune system of the host. Nevertheless, some viral infections like influenza and HIV recur. The recurrence occurs due to the production of virions that are resistant to the neutralizing antibodies that were able to effectively block the infection.
These virions can infect survivors of the acute infection caused by the
original virus. These viruses have a structural plasticity that enables them to tolerate changes in amino acids in their structural proteins while still
retaining their infectivity. There is a lot of diversity in the ability of viruses to exhibit such plasticity. They can range from as little as 3 serotypes as in poliovirus to nearly 100 serotypes in rhinovirus. Consequently, vaccines against poliovirus, measles and yellow fever confer long-term immunity while a new influenza vaccine is needed every year.
Influenza virus
The antigenic properties of influenza viruses are determined by
both hemagglutinin and neuraminidase. Specific host proteases cleave the single peptide HA into two subunits HA1 and HA2. The virus becomes
highly virulent if the amino acids at the cleavage sites are lipophilic.
Selection pressure in the environment selects for antigenic changes in the antigen determinants of HA, that includes places undergoing adaptive evolution and in antigenic locations undergoing substitutions, which
ultimately results in changes in the antigenicity of the virus. Glycosylation of HA does not correlate with either the antigenicity or the selection pressure.
[15] Antigenic variation may be classified into two types, antigenic drift that results from a change in few amino acids and antigenic shift which is the outcome of acquiring new structural proteins. A new vaccine is required every year because influenza virus has the ability to undergo antigenic drift.
Antigenic shift occurs periodically when the genes for structural proteins are acquired from other animal hosts resulting in a sudden dramatic change in viral genome. Recombination between segments that encode for
hemagglutinin and neuraminidase of avian and human influenza virus segments have resulted in worldwide influenza epidemics called pandemics such as the Asian flu of 1957 when 3 genes from Eurasian avian viruses were acquired and underwent reassortment with 5 gene segments of the circulating human strains. Another example comes from the 1968 Hong Kong flu which acquired 2 genes by reassortment from Eurasian avian viruses with the 6 gene segments from circulating human strains.
