What are the roles of CD4 and CD8 in T cell activation?

CD4 is found on the surface of helper T-cells, while CD8 is found on the surface of cytotoxic “killer” T-cells. CD4 and CD8 both bind to MHC (major histocompatibility complex) molecules. The role of MHC is to bind antigens since T-cell receptors themselves cannot directly bind antigens. Thus, MHC is necessary for antigen recognition. CD4 binds to MHC class II molecules, which are only found on antigen-presenting cells (APCs), while CD8 binds to MHC class I molecules, which are found on all nucleated cells.