CD4 antigen is found on a T-cell subset (helper/inducer) representing 45% of peripheral blood lymphocytes.
CD4 is also present on 80% of thymocytes and at a lower level on monocytes.
CD4 is involved in recognition of antigen presented along with MHC class II by APCs.
CD4 is the primary cellular receptor for the human immunodeficiency virus (HIV).