The CD4 antigen is highly expressed on human T helper cells and thymocytes, and at lower levels on monocytes and dendritic cells. This integral membrane glycoprotein of approximately 58 kDa contains four external domains (D1 to D4) that show homology to members of the immunoglobulin (Ig) superfamily. It is responsible for the recognition of the MHC class II antigen. The CD4 antibody recognizes most thymocytes and about 65% of peripheral blood T cells.