ABO and COVID-19: To rule out the possibility that the IgM ALAb detected in COVID-19 patients were directed against ABO antigens, we re-tested, this time against type O PBMC, 10 HD plasma samples and 20 COVID-19 samples previously found to be IgM ALAb positive (n=16) or negative (n=10) for IgM ALAb, against type 0 PBMC.