CD4, also known as T4, is a 55 kD single chain transmembrane glycoprotein and belongs to immunoglobulin superfamily. CD4 is found on most thymocytes, a subset of T cells and at low level on monocytes/macrophages.CD4 is a co-receptor involved in immune response (co-receptor activity in binding to MHC class II molecules) and HIV infection (CD4 is primary receptor for HIV-1 surface glycoprotein gp120). CD4 regulates T-cell activation, T/B-cell adhesion, T-cell diferentiation, T-cell selection and signal transduction.