PROS1 and COVID-19: Another suggested mechanism is the inhibition of the virus adhesion to the host cell via two ways either by anti-A antibody binding to the SARS-COV-2 S protein, and blocking the interaction between this protein and ACE 2 receptors to prevent the virus from lung entry, or via SARS-COV-2 glycan antigen, which is similar to antigen A but does not exist in the O blood group, so it is proposed that the patients with the A blood group have a greater risk of COVID-19 severity as they lack anti-A antibodies (16).