A monoclonal antibody reagent specific for a 'pan T cell' or 'pan B cell' cell surface antigen can be used to identify and
enumerate mature T or B lymphocytes, respectively. A monoclonal antibody reagent specific for a particular cell
surface antigen can also be used in defining the maturational (differentiation) and/or functional state of a lymphocyte
population. The present test utilizes the monoclonal antibodies CD3 (IgG1) and T4 to identify and enumerate peripheral blood T and inducer T lymphocytes via specific binding to the 'pan T cell' and 'inducer T cell' cell surface antigens, CD3 and CD4,respectively. In addition, the test allows distinct lymphocyte populations to be measured simultaneously in the same whole blood sample using a single reagent.