CD4, symbol for a glycoprotein expressed on the surface of helper T lymphocytes and other immune cells. CD4 serves as a receptor for human immunodeficiency virus envelope glycoprotein gp120. Binding of the viral glycoprotein gp120 to CD4 is the first step in viral entry, leading to the fusion of viral and cell membranes. CD4 count is used as an indicator of treatment timing in HIV patients. Abbreviation for cluster of differentiation 4.
