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Antigenic variation is a shift in surface antigens on an infectious organism to help the organism evade the immune systems of potential hosts. Organisms use a variety of tactics for changing the composition of the antigens on their surface. This evolutionary trick allows them to continue growing and spreading in populations, perpetuating their existence. Researchers have an interest in how this works because it can play a role in how people become reinfected with disease. Antigenic variation is also of interest for people in charge of developing vaccines and medications to prevent and treat infection.
Organisms like viruses, bacteria, and parasites all have an external envelope, with a series of surface proteins. When an organism enters a host for the first time, the immune system does not recognize any of the proteins and may allow the organism to multiply, creating an infection. The immune system will learn that those proteins are dangerous, and when the organism appears in the future, the body will go on the attack. It sees the proteins, recognizes them as a threat, and sends out immune cells to kill the organism.
Without antigenic variation, infectious organisms would quickly become extinct. Numbers of vulnerable people in the population would drop and the organisms would not be able to survive. If, however, the organism can change the proteins in future generations, it can adapt and start evading the immune system again. This may happen between hosts, but it can also happen during an active infection. People often notice cyclical patterns in infections where they start to get better, become much worse, and then get better again. This is the result of antigenic variation in multiple generations of organisms growing inside the body.
Some organisms experience random mutations, which can occur at any time. Others actually program in antigenic variation. These organisms have proteins like the license plates on one of James Bond's cars; they can switch proteins on and off to present a completely different plate number, as it were, to the immune system. As they cycle through the different surface antigens, some hosts may resist them because they have been exposed in the past, but others will be vulnerable to infection.
Antigenic variation can happen through recombination, inversion, deletion, and other tricks of DNA. Some organisms are better at it than others. The influenza viruses are a notorious example; they change so much that people must design a new vaccine every year to inoculate people against the flu. Likewise, the HIV virus mutates very rapidly and randomly, making it difficult to prevent or treat because it is a moving target.