The role of CD4 in T-cell activation: accessory molecule or co-receptor?

CD4 is an MHC class II binding protein found on T cells that recognize peptide fragments of protein antigens bound to MHC class II molecules. In this review, Charlie Janeway argues that CD4 is a physical component of the T-cell receptor, and that CD4 augments signalling via the receptor by about 100-fold. He proposes the term co-receptor to describe this molecule and its functional homologue, CD8.