Platelet-activating factor (PAF)-induced chemotaxis and PAF binding to human eosinophils

Compared to PAF-induced platelet aggregation, the chemotactic response to PAF of human eosinophils requires higher PAF concentrations, suggesting a lower binding affinity of PAF for the eosinophil. To test this possibility, we examined the binding of PAF by human eosinophils. The dose range of unlabeled PAF required to produce inhibition of [3H]PAF binding was roughly the same as that for induction of chemotaxis, and was much higher than that in platelets as reported in the literature. These results indicate that the affinity of PAF for the eosinophil is considerably less than that for the platelet, and may explain the higher EC50 for eosinophil chemotaxis.