Plasma IgG Concentrations

Plasma IgG levels appear to influence platelet activation and aggregation by HIT sera. With a platelet-rich plasma (PRP) aggregation test to detect HIT antibodies, Chong et al. (1993a) showed variable platelet sensitivity to aggregation that was stable over time among different platelet donors. Chong and coworkers showed that the addition of purified human IgG to the PRP inhibited platelet aggregation by HIT sera, with complete inhibition at 40 mg/mL. It is possible that the effect of purified IgG is due to the presence of small IgG oligomers, because Karas et al. (1982) demonstrated that monomeric IgG does not bind to the platelet FcyRIIa. Furthermore, Greinacher et al. (1994b) showed that different preparations of intravenous IgG (ivIgG) for therapeutic use varied in their ability to inhibit HIT antibody-induced platelet serotonin release. Although the use of ivIgG to treat HIT does not appear to be common, it has some rationale in certain clinical settings (see Chapter 12).

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