This system allows lymphocytes to identify and destroy cells in which viruses or bacteria are hidden and multiplying. Normally, the trillion of lymphocytes do not attack their host. To prevent such attacks, lymphocytes bearing receptors that might react with host tissues are selectively destroyed during their development. Some organisms have evolved ways of evading or subverting the body's defenses. The malaria parasite, for example, changes its coat proteins to stay one step ahead of the host's immune cells.
Herpes viruses become almost undetectable to lymphocytes.
The AIDS virus destroys a subset of T cells that are essential for a successful immune response. Thus, the immune system fights off many but not all infections. By learning more about how such pathogens work, scientists may find new ways to thwart them.