AB/O-mismatch–induced lysis of human RBCs causes platelet activation. Washed erythrocytes of a donor with AB-group blood were added to hirudin-anticoagulated whole blood of a donor with group O blood in absence or presence of ravulizumab and OmCI (both at final concentration of 0.8 μM). (A) Surface expression of the platelet activation marker CD62P was determined via flow cytometry, and hemolytic activity was determined in the respective plasma samples measuring released hemoglobin from the supernatant. (B) The level of complement activation in the reaction mixtures was investigated by measurement of C3a and C5a concentrations after the reactions had been stopped by the addition of EDTA. Data sets were either analyzed using a Prism mixed-effects model (in case of missing values) or repeated measures one-way ANOVA. Experimental groups were post hoc tested for statistical significance with Tukey correction for multiple comparisons. For the sake of visibility, nonsignificant P values were omitted from graphs, unless they were of relevance to the experimental hypotheses.