A receptor molecule on the surface of a cell that enhances the activity of another receptor following binding of its extracellular ligand. For example, optimal activation of T cells in an immune response depends on the involvement of the relevant coreceptors in a cluster with the T-cell receptors. The coreceptors are CD4 or CD8 proteins, which lie alongside the T-cell receptor in the plasma membrane. Only when both coreceptor and receptor bind the MHC-antigen complex simultaneously is the full set of intracellular effector molecules recruited and the signal pathway inside the cell maximally activated. Similarly, B-cell receptors also require the contributions of several coreceptors, notably CD19, CD21, and CD81 proteins, for full activation of intracellular signalling pathways. See also CD.