T-cell surface antigen CD4 <p>The CD4 glycoprotein on the surface of T cells participates in the immune response and is the receptor for HIV infection. The structure of a soluble fragment of CD4 has been determined to 2.3 A and reveals that the molecule has two intimately-associated immunoglobulin-like domains connected by a continuous beta strand. Residues implicated in HIV recognition reside in domain D1. Domain D2 is distinguished by a variation in the beta-strand topologies of antibody domains that results in a truncated beta-barrel with a non-standard intra-sheet disulphide bond [<cite idref="PUB00004084"/>, <cite idref="PUB00004083"/>]. The binding sites for monoclonal antibodies, class II major histocompatibility complex molecules, and HIV gp120 can be mapped on the molecular surface.</p>