CD4，a single chain transmembrane glycoprotein，is found on a T cell subset （helper/inducer） representing 45% of peripheral blood lymphocytes. It is also present on 80% of thymocytes and at a lower level on monocytes. It is involved in recognition of antigen presented along with MHC class II by APCs. It serves as receptor for HIV. Antibody to CD4 recognizes T-helper cells required for recognition of class II antigens. It reacts with ~60% of peripheral blood E rosette-positive （E+） cells while showing negligible reactivity with E- cells，monocytes，granulocytes，EBV-transformed B cell lines，and mouse splenocytes.
