T cell receptor, CD4 and MHC proteins. Illustration of a cell membrane with a T cell receptor (dark blue, centre) in complex with an MHC (major histocompatibility complex) class II-peptide complex (purple). The T cell receptor activates the immune response to antigens in T cells (a form of lymphocyte). The antigen (light green) is a peptide from a tumour cell, bacteria or virus. At centre left is a CD4 protein (light blue). 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. The CD4 (cluster of differentiation 4) protein binds to the MHC complex and helps the T cell to recognise a threat and initiate an immune response.