Molecular evolution of the vertebrate immune system.

An understanding of the evolution of vertebrate immunity is slowly emerging from studies of chordates that share distant with mammals. In higher vertebrates, such as birds and mammals, we know that two receptor systems are operative. B cells use immunoglobulins to bind foreign agents (the functionally defined antigens). T cells use T-cell receptors (TCRs) to respond to antigen in the form of processed peptides bound to cell surface proteins encoded in the major histocompatibility complex (MHC). Thus, for T cells, two receptor molecules are required for recognition of antigen. First, the MHC molecule on the infected cell binds the processed antigenic peptide; second, the TCR binds the MHC molecule-antigenic peptide complex.