TCR-APC Interaction

Antigen Presenting cells express both MHC molecules,in this case MHC II molecule co stimulator molecule such as CD80.Stimulation of T cells in this case CD4 tcell by the antigen presenting cell involves the interaction of T cell receptor and co receptor molecule with the MHC peptide complex.As well as the interaction between the  CD80 and CD28.The interaction of antigen presenting cell with T cell causes signals to pass in both direction.Signalling the antigen presenting cell to express additional co-stimulatary molecules such as CD86 and CD40.At the same time signals to T cell receptor and CD28 induce T cell to express CD40L(CD40 ligand).the interaction between CD40L and CD40 As well the additional stimulation through CD28 mediated by interaction with CD86 resolved in full activation of CD4 T cell.

Activation of CD8 T cell also requires multiple receptor ligand interaction ,the same activation siganals induce the antigen presenting cell to express other co-stimulatary molecules such as 4-1BBL ligand (4-1BBL),while initial activation of  CD8 T cells induces the expression of 4-1BB,Binding of 411B ligand to 41BB is thought to required for full activation of CD8 T cells.