CD4 is a 55kD glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. It was originally known as leu-3 and T4 before being renamed in 1984. Anti-CD4 is used in the immunophenotyping of reactive lymphocytes and lymphoproliferative disorders. The majority of peripheral T cell lymphomas are derived from the helper T cell subset so that most post-thymic T cell neoplasms are CD4+ CD8-. As with other T cell antigens, CD4 may be aberrantly deleted in neoplastic T cells so that the evaluation of such tumors requires the application of a panel of markers in order to identify tumors with such anomalous antigenic expression.