 It is the specific antigen or epitope that is loaded into the MHC determines the antigen-specificity.The peptide-loaded MHC engages with the cognate T cell receptor (TCR) found on the T cells.T cells need another signal to become activated in addition to Signal 1, this is done by co-stimulatory molecules such as the proteins CD80 (B7.1) or CD86 (B7.2), although other additional co-stimulation molecules have been identified.