Invitrogen's CD3 monoclonal antibody recognizes the CD3 antigen). The monoclonal antibody may be used to enumerate mature T lymphocytes in human peripheral blood. This monoclonal antibody may also be used, in combination with other indicators, for the diagnosis or prognosis of some immunodeficiency diseases, including agammaglobulinemia and the severe combined immunodeficiency disease (SCID), which exhibit decreased percentages of T lymphocytes. Invitrogen's CD4 monoclonal antibody recognizes the CD4 antigen. This antibody may be used, in combination with other indicators, for the diagnosis or prognosis of immunodeficiency diseases, including hypogammaglobulinemia, severe combined immunodeficiency (SCID) and the acquired immunodeficiency disease (AIDS). The CD4 antigen domain is generally recognized to contain the binding site for the HIV retrovirus through interaction with the viral envelope glycoprotein.