The CD4 antigen is a single-chain transmembrane glycoprotein with a 59 kDa molecular weight. CD4 binds to a non-polymorphic region of MHC Class II molecules and is a co-receptor in MHC Class II restricted antigen-induced activation. CD4 is expressed on the T "helper" lymphocytes. It is present on most thymocytes where it is frequently co-expressed with CD8. CD4 is also expressed on all monocytes, although with a lower density than on T lymphocytes. CD4+ T lymphocytes are active in inducing and helping the synthesis of immunoglobulins by B cells. CD4 is a receptor for the Human Immunodeficiency Virus type I (HIV-1) envelope protein gp120.