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Our immune system is never idle. Their task is to detect and eliminate invasive pathogens, and they have no time to lose. The adaptive immune system identifies infectious organisms by recognizing foreign proteins on the surfaces of bacteria, viruses and unicellular protozoans. The interaction of these antigens with immune cells triggers a series of downstream events, which in most cases leads to the elimination of the pathogen.
But pathogenic organisms have developed strategies that enable them to escape detection by the immune system, and the strategies employed by remotely related organisms are often remarkably similar to each other. One way of confusing the immune system is to increase the structural heterogeneity of the antigens it encounters. In bacteria,pathogenic yeast and parasites this can be done by randomly activating different members of gene families, which code for non-identical versions of the proteins expressed on their surfaces. This strategy essentially a