The number of discrete hemolytic foci and of hemolysin-forming cells arising in the spleens of heavily irradiated mice given sheep erythrocytes and either syngeneic thymus or bone marrow was not significantly greater than that detected in controls given antigen alone. Thoracic duct cells injected with sheep erythrocytes significantly increased the number of hemolytic foci and 10 million cells gave rise to over 1000 hemolysin-forming cells per spleen. A synergistic effect was observed when syngeneic thoracic duct cells were mixed with syngeneic marrow cells: the number of hemolysin-forming cells produced in this case was far greater than could be accounted for by summating the activities of either cell population given alone. The number of hemolytic foci produced by the mixed population was not however greater than that produced by an equivalent number of thoracic duct cells given without bone marrow. Thymus cells given together with syngeneic bone marrow enabled irradiated mice to produce hemolysin-forming cells but were much less effective than the same number of thoracic duct cells. Likewise syngeneic thymus cells were not as effective as thoracic duct cells in enabling thymectomized irradiated bone marrow-protected hosts to produce hemolysin-forming cells in response to sheep erythrocytes. Irradiated recipients of semiallogeneic thoracic duct cells produced hemolysin-forming cells of donor-type as shown by the use of anti-H2 sera. The identity of the hemolysin-forming cells in the spleens of irradiated mice receiving a mixed inoculum of semiallogeneic thoracic duct cells and syngeneic marrow was not determined because no synergistic effect was obtained in these recipients in contrast to the results in the syngeneic situation. Thymectomized irradiated mice protected with bone marrow for a period of 2 wk and injected with semiallogeneic thoracic duct cells together with sheep erythrocytes did however produce a far greater number of hemolysin-forming cells than irradiated mice receiving the same number of thoracic duct cells without bone marrow. Anti-H2 sera revealed that the antibody-forming cells arising in the spleens of these thymectomized irradiated hosts were derived, not from the injected thoracic duct cells, but from bone marrow. It is concluded that thoracic duct lymph contains a mixture of cell types: some are hemolysin-forming cell precursors and others are antigen-reactive cells which can interact with antigen and initiate the differentiation of hemolysin-forming cell precursors to antibody-forming cells. Bone marrow contains only precursors of hemolysin-forming cells and thymus contains only antigen-reactive cells but in a proportion that is far less than in thoracic duct lymph.
