The T cell receptor (TCR) for antigen is a membrane-bound member of the immunoglobulin superfamily (IgSF) that recognizes a complex of antigen bound to self major　histocompatibility complex (MHC) molecules expressed on the cell-surface of antigen-presenting cells (APC). Both cytotoxic and helper T cells typically respond to antigen which is associated with different types of MHC molecules, class I and II, respectively. CD4 is a 55-60 kDa cell-surface glycoprotein, which participates in the molecular complexes involved both in T cell development and its antigen recognizing activity, by binding to the nonpolymorphic region of class II MHC. This interaction results in increased intercellular adhesion and enhanced stimulation of T cells by signal transduction which is mediated by intracellular molecules. CD4 is also considered as a stage marker of T cell development in thymus, for it is expressed on the cell surface in a stage specific manner, during T cell development.