= Co-receptor
A \b[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.<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.]{ref}
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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