Hypothesis: differentiation of the human lymphoid system based on cell surface markers.

Human lymphocytes can be separated into distinct populations based upon receptors on their cell surface. Thymus-derived (T-cell) lymphocytes can be identified by their ability to form rosetts with sheep erythrocytes (SRBC); bone marrow-derived (B-cell) lymphocytes bear characteristic surface markers for immunoglobulin, complement, and the Fc portion of IgG. Recently, populations of lymphocytes having either multiple markers or no detectable markers (null cells) have been observed. Based on studies of cell surface markers, a scheme is proposed that expands the known differentiation of the lymphod cell to include subpopulations which represent developmental stages. It is suggested that lymphocyte maturation involves alloantigenic changes in a circulating stem cell-drived nill cell, leading to a cell bearing markers for both T- and B-cells. It is from this latter cell that the classic T- and B-cells ultimately arise. Maturational defects which may explain the origin of primary lymphoproliferative diseases are discussed.