The OKT4 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 OKT4 and the RPA-T4 monoclonal antibodies recognize different epitopes of CD4 and they do not exhibit cross-block binding.