Marc Jenkins
Department of Microbiology
Center for Immunology
University of Minnesota

The mammalian immune system must specifically recognize and eliminate foreign invaders, but refrain from damaging the host itself. This task is accomplished in part by the production of a large number of T lymphocytes each bearing a different antigen receptor to match the enormous variety of antigens present in the microbial world. However, because antigen receptor diversity is generated by a random mechanism, the immune system must tolerate the function of T lymphocytes that by chance express a self-reactive antigen receptor. Therefore, during early development, T cells that are specific for antigens expressed in the thymus are physically deleted. The population of T cells that leaves the thymus and seeds the secondary lymphoid organs contains helpful cells that are specific for antigens from microbes, but also potentially dangerous T cells that are specific for innocuous extrathymic self antigens. The outcome of a peripheral T cell's encounter with these two types of antigens is to a great extent determined by the inability to naive T cells to enter non-lymphoid tissues, or to be productively activated in the absence of inflammation.

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