Effect of plasma from patients with severe COVID-19 on washed healthy platelet aggregation response. Washed platelets from 6 healthy donors were resuspended in plasma from 11 to 15 patients with severe COVID-19 or 9 healthy donors. The platelet aggregation response (% of maximal platelet aggregation) was then assessed by light transmission aggregometry in response to low doses of CRP (0.075 µg/mL) (A-B) and TRAP (25 µM) (C-D) for 10 minutes. Representative aggregation traces are shown (A, C) as well as the quantification of maximal platelet aggregation response (B, D). Results are mean ± SEM; each circle represents an individual plasma. **P < .01 according to the nonparametric Mann-Whitney test. The potential impact of FcγRIIA receptor on the potentiation of plasma of patients with severe COVID-19 on CRP-induced platelet aggregation was assessed by using the IV.3 neutralizing antibody (E). As a positive control of the IV.3 antibody efficiency, 3 sera from patients with heparin-induced thrombocytopenia (HIT) were used in the presence of heparin (0.5 IU/mL) to induce healthy platelet aggregation via FcγRIIA. The specificity of the reaction was assessed by addition of a large excess of heparin to impair platelet aggregation induced by HIT sera (F).