PLATELET-AGGREGATION AND HISTAMINE-RELEASE BY IMMUNOLOGICAL STIMULI

Platelet aggregation and histamine release were evaluated in normal an d IgE pretreated human platelets exposed in vitro to IgE, anti-IgE and thrombin. The response of platelets from atopic donors directly stimu lated with anti-IgE was also evaluated. Histamine release was measured by fluorimetric analysis of histamine content in platelets and in sup ernatants. The morphology of platelets exposed to immunological and no n-immunological stimuli was recorded using an electron microscope. A d etectable amount of histamine was measured in quiescent platelets. The ir exposure to varying concentrations of thrombin produces a progressi ve aggregation which runs parallel to histamine release. The effects w ere significantly enhanced in platelets pretreated with IgE. Incubatio n of normal platelets with increasing concentrations of IgE myeloma pr otein, or with anti-human IgE antibody was ineffective on both aggrega tion and histamine release. However, incubation of platelets passively sensitized with IgE-myeloma protein with different concentrations of anti-human IgE antibody produces a concentration-dependent increase bo th in aggregation and histamine release. The same effects were obtaine d using platelets from atopic donors directly stimulated with anti-IgE . The electron microscopic pattern of platelet aggregation induced by thrombin was indistinguishable from that evoked by anti-IgE in IgE pre treated platelets. Loratadine, a non-sedative H-1-receptor blocker, si gnificantly abated platelet aggregation and histamine release induced by anti-IgE in IgE pretreated platelets.