A co-receptor is a cell surface receptor that binds a signalling molecule in addition to a primary receptor in order to facilitate ligand recognition and initiate a biological process, such as entry of a pathogen into a host cell.
The co-receptor may be a required component of the ligand complex, or may just increase sensitivity of the receptor.
Lymphocyte Development (Pillai) defines a co-receptor as a 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 co-receptor. Signaling molecules associated with the coreceptor may "turn on" the antigen receptor.
