The CD3 complex is a group of cell surface molecules that associates with the T-cell antigen receptor (TCR) and functions in the cell surface expression of TCR and in the signaling transduction cascade that originates when a peptide:MHC ligand binds to the TCR. CD3 is composed of invariant subchains belonging to the immunoglobulin superfamily. It is found on the surfaces of T cells and of thymocytes.