Co-receptor

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. As a result, an antigen may physically bring together an antigen receptor and a coreceptor. Signaling molecules associated with the coreceptor may "turn on" the antigen receptor.

Many antigens, including HIV, require a receptor/coreceptor complex in order to infect the target cell.