The veto phenomenon and T-cell regulation.

Lymphocytes bearing anti-self receptors are produced during ontogeny, yet normally the immune system does not actively react against self components. Instead, it is thought that selfreactive cells are either deleted or, failing this, suppressed by regulatory networks similar to those limiting the immune response against non-self determinants. Recognition events between T cells play a central role in both mechanisms. Here Rick Miller describes 'veto cells' which can inactive (apparently permanently) T cells which recognize them. Some T cells can themselves serve as veto cells. The veto mechanism has been shown capable of eliminating T cells reactive against self class I major histocompatibility complex (MHC) antigens and against antigen in association with either class I or class II MHC products.