Posttransfusion purpura: conversion of PLA1-negative platelets to the PLA1-positive phenotype by stored plasma is not due to the presence of soluble PLA1 antigen.

Three previous articles have reported a PLA1 antigen in the plasma of stored blood which is capable of binding to PLA1-negative platelets in the presence of divalent cations, rendering them PLA1 positive. Such a mechanism could explain the enigma of posttransfusion purpura (PTP), i.e., severe thrombocytopenia in a healthy subject with PLA1-negative platelets secondary to the infusion of blood containing PLA1-positive platelets. We find that the PLA1 antigen of stored blood is due to the presence of platelet fragments which can be removed by centrifugation and that divalent cation-chelating agents play no role in the apparent binding of these fragments to platelets. The apparent conversion of PLA1-negative platelets to the PLA1-positive phenotype by incubation with stored plasma from a PLA1-positive subject is due to the cosedimentation of platelet fragments with the platelets. No soluble PLA1 antigen was found in the plasma of five patients with acute posttransfusion purpura.