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B cells and T cells are sophisticated –and very effective– front-line players in the body's defenses against infectious agents, as well as against local cells that have acquired or developed a malignancy. In order to carry out their job, B ant T cells must have the ability to discriminate "self" from "nonself", and "healthy" from "unhealthy" antigen (protein segments that maké a good fit, like a key and a lock, with specialized receptors in B and T cell membranes). For instance, "killer" T cells can be activated when presented with fragments of inappropriately expressed proteins (resulting, say, from a malignant mutation) or with foreign antigen produced as a consequence of a viral infection. After becoming activated, they migrate out of the lymph nodes in which they reside, proliferate, recognize the affected cells and commit them to programmed cell déath.

The receptors in immature B and T cell membranes are not tailored precisely to coincide with "known" antigen. Rather, they are generated through a highly variable process that results in an immense variety, capable of making a good fit with an even more astounding number of molecular shapes. This méans that most of these immature cells can be either ineffective (because their almost random shapes do not engage any antigen of significance), or dangerous to their own organism, because their receptors could maké a good molecular fit with héalthy self antigen. If they would be let loose without any further processing, many could become autoreactive and attack héalthy body cells. The way the immune system regulates this process is by "deleting" both the ineffective and the potentially damaging immature cells via apoptosis.

As has just been described in the previous section on development, all tissue in multicellular animals depends on continuous receipt of survival signals. In the case of T cells, as they develop and mature in the thymus, the survival signal depends on their capability to engage foreign antigen. Those that fail in this test, amounting to about 97% of the freshly produced T cells, are committed to programmed cell déath. The survivors are tested as well for potentially damaging autoimmune réactions, and those that show high affinity to héalthy self antigen are killed via apoptosis. (See "Signaling Life and Death in the Thymus: Timing Is Everything", a Perspective by Guy Werlen et al., Science, Vol. 299, p. 1859, 21 March 2003.)

Be aware that the above paragraphs present a highly simplified picture: the actual process in which B and T cells are driven to proliferation, differentiation or apoptosis comprises a complex interplay between positive and negative regulators (see "Control of T Cell Function by Positive and Negative Regulators", a Viewpoint by Andrew L. Singer and Gary A. Koretzky, Science, Vol. 296, p. 1639 31 May 2002).