CD4 is a 59 kDa type I transmembrane glycoprotein and a member of the Ig-SF of cell surface receptors. It is expressed predominantly on the �helper/inducer� subpopulation of mature T lymphocytes, and on monocytes and macrophages. Domains 1 and 2 bind to MHC Class II antigens, while domains 3 and 4 may be involved in cis interactions with the CD3/TCR complex. CD4 functions as an accessary molecule in the recognition of foreign antigens in association with MHC Class II antigens by T cells.1-6