Blocking antibody directed against donor responding cells has been found to develop in HL-A-mixed lymphocyte culture-identical donor-recipient combinations after renal transplantation in which splenectomy was not perfortned. Cytotoxic antibody associated with rejection episodes and detected only by the discriminatory mixed lymphocyte culture developed in other HL-A-mixed lymphocyte culture-identical combinations as well. It disappeared as blocking antibody appeared and the post-transplant course became uneventful. In addition, both cytotoxic and blocking activity were shown to develop and coexist in a patient who received two successive renal transplants: Cytotoxic against donor 1 (kidney rejected), and blocking against donor 2 (kidney accepted). These findings are taken to be strong evidence in favor of qualitatively demonstrable separate antibody molecules dealing with two separate functions, i.e., cytotoxicity versus enhancement or immunoregulation.
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