The role of Class I and II antigens in T cell recognition

T and B lymphocytes each exhibit a large number of specificities in respect of the recognition of antigen that are clonally inherited. They differ, however, in that T cells recognise cell bound antigen in association with a gene product of the major histocompatibility complex (the principle of MHC restriction) whereas recognition by B cells is generally MHC unrestricted. Thus, the modern concept of class I and II antigen function is that they act as guidance systems for T cells. Two basic models, involving one or two binding sites for antigen and MHC product, have been proposed to account for MHC-restricted recognition of antigen by T cells. Several lines of evidence, including detailed analysis of the structure of the T cell receptor, favour the one receptor model. This receptor (when expressed on the surface of a T ‘killer’ cell) apparently recognises the two N-terminal domains of class I antigens and, at least in some cases, processed antigen in the form of peptide fragments.