The growth and function of bone marrow, thymus, and spleen cell suspensions were studied after injection into irradiated recipients. Parental and hybrid mice (C57L, A/He, and C57L X A/He) were used as both donors and recipients. Marrow cell function was shown by enlargement of the recipient spleen. Thymus cell function was indicated by the enhanced antibody production to sheep RBC by combinations of thymus and marrow cells. Spleen cell function was shown by production of antibodies to sheep RBC. Both marrow and spleen cells showed “hybrid resistance”, i.e., they functioned better when donor and recipient were syngeneic (either parent → parent, or F1 → F1) than when they were allogeneic (parent → F1 or F1 → parent) Similarly, C57BL thymus cells functioned well in C57L but not in LAF1 hybrid recipient strains. Thymus cells, on the contrary, functioned as well in LAF1 as in A recipients. Growth of F1 marrow was inhibited by parental thymus cells only in syngeneic parental recipients and not inrecipients. This may represent the immunological reequipment of irradiated parental hosts by parental thymus cells. Hybrid resistance was not seen if parental spleen cells were enclosed with antigen in Millipore diffusion chambers and placed in F1 recipients. This suggests that hybrid resistance depends upon contact between parental and F1 cells. © 1969 by The Williams and Wilkins Co.
