A co-receptor is a receptor that acts with another receptor in a specified recognition, signaling or entry process. The word describes a functional partnership: which partner is needed depends on the biological system and the measured response. It does not mean that every receptor involved must have the same normal ligand or physiological role.
A revealing example is entry by certain HIV-1 isolates. H. Deng and colleagues reported in 1996 that CD4 alone did not explain entry requirements. The chemokine receptor then called CC-CKR-5, now CCR5, supported entry mediated by the envelopes of tested primary macrophage-tropic HIV-1 strains. The comparison with T-cell-line-adapted viruses showed why a co-receptor must be identified in a particular virus–cell context.[ref]
Identification of a major co-receptor for primary isolates of HIV-1 by H. Deng and others. Abstract, Nature 381 (1996), pp. 661–666; DOI 10.1038/381661a0.
The conceptual experiment separates recognition from sufficiency: possessing one required cell-surface receptor does not ensure that a cell has all the components needed for entry. Nor does identifying entry cofactors by itself establish every later step of a productive infection.
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