What’s Antigenic Variation?

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Antigenic variation is a process where infectious organisms change their surface antigens to evade the immune system of potential hosts, allowing them to continue to grow and spread. This process is of interest to researchers studying reinfection and to those developing vaccines and drugs. Antigenic variation can occur through random mutations or programmed changes, and can happen between hosts or during an active infection. Influenza and HIV are examples of organisms that are particularly good at antigenic variation.

Antigenic variation is a movement of surface antigens onto an infectious organism to help the organism evade the immune system of potential hosts. Organisms use a variety of tactics to change the composition of antigens on their surfaces. This evolutionary trick allows them to continue to grow and spread in populations, perpetuating their existence. Researchers are interested in how this works because it may play a role in how people reinfect themselves with the disease. Antigenic variation is also of interest to people tasked with developing vaccines and drugs to prevent and treat infection.

Organisms such as viruses, bacteria and parasites all have an outer shell, with a variety of surface proteins. When an organism first enters a host, 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 proteins, recognizes them as a threat, and sends out immune cells to kill the organism.

Without antigenic variation, infectious organisms would quickly become extinct. The number of vulnerable people in the population would decrease 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 can 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, get much worse, and then get better again. This is the result of antigenic variation over multiple generations of organisms growing within 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 turn proteins on and off to present a completely different number of plates, so to speak, to the immune system. As they pass through the different surface antigens, some hosts may resist because they have been exposed in the past, but others will be vulnerable to infection.

Antigenic variation can occur through recombination, inversion, deletion and other DNA tricks. Some organisms are better at this than others. Influenza viruses are a well-known example; they change so much that people have to design a new vaccine every year to vaccinate people against the flu. Similarly, the HIV virus mutates very rapidly and randomly, making prevention or treatment difficult because it is a moving target.

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