Immunochemical Characterization of a Human B Lymphocyte Differentiation Antigen (p65)

A human B‐cell differentiation antigen (BDA‐1) with a molecular weight of 65,000 was Identified by use of an anti‐B‐cell xenoantiserum. BDA‐1 was isolated by immunoprecipitation of cell lysates from a B‐lymphoblastoid cell line (SB) and from blood B lymphocytes (sIg⁺ER⁻) of four normal individuals. It was not detectable in cell membrane lysates from two T‐lymphoblastoid cell lines (HSB‐2 and MOLT‐3) or from enriched normal T cells (sIg⁻ER⁺). BDA is a singlechain molecule since its migration in SDS‐PAGE gels was not altered by heating the protein to 100 C or by treatment with 2‐mercaptoetbanol. Immunodepletion experiments demonstrated that the antigen recognized by anti‐BDA xenoantiserum has neither structural nor antigenic relationships with la‐like antigen or β‐microglobulin.