T-cell receptor in complex with the MHC class II-peptide complex. The antigen (light green) is a peptide from a tumor cell, bacteria or virus. Dendritic cells present antigens to lymphocytes through their membran bound MHC-molecules (violet). CD4 molecules (light blue) bind to other portions of the MHC, strengthening the interaction. After binding to the MHC-antigen complex, The T-cell receptor (blue) sends a signal cascade through an attached G-protein into the T-lymphocyte cell, that activates an immune response.