CD4, a member of the IgG superfamily, is a cell surface receptor expressed on helper T lymphocytes, monocytes, B-lymphocytes and several other cells. In addition to being a receptor for HIV, CD4 participates in the recognition of foreign peptides bound to class II MHC complexes¹.

The virion surface envelope glycoprotein, gp120, consist of five variable domains (V1-V5) and five conserved domains (C1-C5). The high-affinity binding of CD4 to gp120 is believed to involve interactions between the C3 and C4 domains of gp120 and the N-terminal extracellular domain of CD4².

References

1. Dagleish, A.G.; Beverly, P.C.; Clapham, P.R.; et al., The CD4 (T4) antigen is an essential component of the receptor for the aids retrovirus. NATURE 312:763-764 (1984).
2. Arthos, J.; Deen K.C.; Chaikin M.A.; et al., Identification of the residues in human cd4 critical for the binding of HIV. CELL 57:469 (1989).n