CD4 is a cell-surface glycoprotein found on the mature helper T cells and immature thymocytes， as well as on monocytes and macrophages. （Some cytotoxic T cells have CD4 protein as well.） Normally， about 65% of T cells in the blood are CD4+ （have CD4 protein protruding from their membrane）. A mature T cell with either have CD4 or CD8， but not both. During one stage of development T cells develop CD4 and CD8 receptors， but they eventually are differentiated in the thymus and become more specialized.
