1. Ovalbumin administration to animals injected with 0.5-5 x 10(6) washed platelets microliter-1 from actively sensitized animals induced a marked decrease (maximum of around 50% at 60 min) in the number of circulating leucocytes, whereas platelet counts were unaffected. The intensity of the decrease in leucocyte counts was dependent upon the concentration of the injected platelets. 2. No drop in leucocyte counts was measured upon antigen challenge of guinea-pigs injected with platelets from non-sensitized animals. 3. This phenomenon was unaffected by pretreatment of the recipient animals with a platelet-activating factor (PAF) antagonist WEB 2086 (3 mg kg-1 i.v.) but was markedly reduced (around 50% inhibition) by the anti-allergic drug nedocromil sodium (100 mg kg-1 i.v.). By contrast, indomethacin (5 mg kg-1 i.v.) caused a significant (P less than 0.01) enhancement of the antigen-induced leucopenia, whereas the mixed cyclooxygenase and lipoxygenase inhibitor, BW 755C (10 mg kg-1 i.v.) suppressed the drop in leucocyte counts evoked by ovalbumin administration. 4. These results indicate that platelets from actively sensitized guinea-pigs transferred to normal animals still responds to the specific antigen with the activation of circulating leucocytes. This phenomenon appears to be independent of the production of PAF and of cyclo-oxygenase metabolites. By contrast, the production of the metabolites of the lipoxygenase pathway by platelets could account for the marked leucopenia observed following the immunological stimulation.
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