Abstract

T lymphocytes circulate continually throughout the peripheral lymphoid organs, where they scrutinize the surface of cells to detect the presence of nonself protein fragments. During the last years, many facets of T-cell function have been unravelled. After being bound by major histocompatibility complex (MHC) molecules, peptides derived from nonself as well as from self proteins are delivered to the cell surface. A few copies of a nonself peptide “presented” at the cell surface in the context of an MHC molecule can be detected by specific T cells, and suffice to trigger T-cell activation. This paper reviews the requirements imposed on T cells to fulfill this exquisite sensitivity. BioEssays 20:412–422, 1998.© 1998 John Wiley & Sons Inc.