The RPA-T4 monoclonal antibody specifically binds to the CD4 receptor for the human immunodeficiency virus (HIV). CD4 is a 59 kDa single-chain transmembrane glycoprotein that expressed on the surface of most of the thymocytes, T-helper cells, and in low levels on monocytes and macrophages. CD4 is a co-receptor in the antigen-induced T cell activation, together with the MHC class II.The RPA-T4 antibody is capable of blocking HIV binding and inhibiting syncytium formation, by binding to the D1 domain of the CD4 antigen. The OKT4 and the RPA-T4 monoclonal antibodies recognize different epitopes of CD4 and they do not exhibit cross-block binding.