a second receptor that enables an organism to infect a cell. In addition to the CD4 receptor on cell surfaces, HIV requires the co-receptor fusin to infect T-cells and the co-receptor CKR5 to infect macrophages.

additional receptor molecules that contribute to the T or B cell receptor signalling complex

A co-receptor is a second cell surface receptor required for the entry of a pathogen into a host cell or initiation of a biological process. It is also a protein that increases the sensitivity of an antigen receptor to its antigen by binding to other ligands. From Lymphocyte Development (Pillai), a coreceptor is defined as a signaling receptor that directly associates with the very same antigen that is seen by the antigen receptor.