describe activation of CD4 helper T cells by interaction of the TCR with peptide fragments presented by class II MHC molecules on APCs
The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex, on the surface of the APC. This triggers initial activation of the T cells. The CD4 and CD8 molecules then bind to the MHC molecule too, stabilizing the whole structure. This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion.