Circulating immune complexes can directly activate platelets (Figure 17) by initiating complement binding [106,107,108], have been observed as a cause of platelet activation in COVID-19-related contexts [109,110,111], often contain PF4-reactive IgG [112,113,114] (and thus bacteria-induced antibodies), and participate in the formation of NETosis [115,116]. The gene discussed is PF4; the disease is COVID-19.