A co-receptor is a cell surface receptor that binds to a signaling molecule, in addition to a primary receptor, in order to facilitate recognition of the ligand and initiate a biological process, such as the entry of a pathogen into a host cell.

The co-receptor can be a necessary component of the complex ligand, or it can simply increase the sensitivity of the receptor.

A co-receptor is defined as a signaling receptor that is directly associated with the same antigen, which is seen by the receptor antigen.As a result, an antigen can physically bind an antigen receptor and a co-receptor. The signaling molecules associated with the co-receptor can "activate" the antigen receptor.

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