The transfusion of blood components (buffy coat and platelets) may induce characteristic alloimmune response or suppressive regulation that have, in certain cases, a beneficial effect on allograft survival. The blocking effect of the sera of donors immunized with platelets on mixed lymphocyte culture and on the response of lymphocytes to mitogen as well as soluble antigen (PPD, tetanus toxoid) stimulation was studied. Six sera from 7 volunteers displayed a strong and significant nonspecific MLC blocking effect that was de-tectable on the 10th day following the second platelet transfusion (PT). Incubation of isolated stimulator and effector cell population with this “blocking sera” showed that the latter are involved in the mediation of suppression in the MLC test. This inhibitory effect is associated with the serum IgG fraction lacking any correlation with either class I or class II specific cytotoxic antibodies. Selective blocking behavior was found on transformation activity induced by mitogens or soluble antigens. Thus, sera of platelet-transfused volunteers decreases the responsiveness of lymphocytes to phytohemagglutinin, while the response to concanavalin A, tetanus toxoid, and PPD was not suppressed. Separate treatment of T and B lymphocyte populations and monocytes with the blocking sera showed that only T and B lymphocytes are targets, and not the monocytes for the inhibition in the case of PHA-induced proliferation. Indirect evidence may support the notion that MLC-inhibiting and FcR- blocking antibodies may be analog products of a regulatory alloimmune response induced by leukocytes that are partially responsible for the beneficial transfusion effect in organ transplantation.
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