professional antigen presenting cells Express both MHC molecules in this case MHC Class 2 and co- stimulator molecules such as cd80 stimulation of a t- cell in this case a CD4 T Cell by the antigen presenting cell involves the interaction of the t- cell receptor and co-receptor molecules with the MHC peptide complex as well as the interaction between cd80 and cd28 the interaction of the antigen presenting cell with the t- cell causes signals to pass in both directions signaling the antigen presenting cell to express additional co- stimulatory molecules such as cd86 and cd40 at the same time signals through the t- cell receptor and cd28 induce the t- cell to express cd40 liand the interaction between cd40 liand and cd40 as well as the additional stimulation through cd28 mediated by interaction with cd86 result in the full activation of the CD4 T Cell Activation of cd8 t- cells also requires multiple receptor Li interactions the same activation signals induce the antigen presenting cell to express other co- stimulatory molecules such as 41bb liand or 41 BBL while the initial activation of cdat cells induces the expression of 41bb binding of 41bb liand to 41bb is is thought to be required for the full activation of cd8 cells