ABO blood group system roles in cell membrane integrity, cell adhesion, membrane transportation of molecules, and acting as receptors for extracellular ligands, and enzymes. ABO antigens are found having similar roles on epithelial cells as well as red blood cells. The ABO antigen is also expressed on the von Willebrand factor (vWF) glycoprotein, which participates in hemostasis (control of bleeding). In fact, having type O blood predisposes to bleeding, as 30% of the total genetic variation observed in plasma vWF is explained by the effect of the ABO blood group, and individuals with group O blood normally have significantly lower plasma levels of vWF
Hh blood group are IgM and IgG (and this seems to have a very important role in the low frequency of hemolytic disease of the newborn among non-Bombay offspring of Bombay mothers). It is very important, in order to avoid any complications during a blood transfusion, to detect Bombay phenotype individuals, because the usual tests for ABO blood group system would show them as group O. Since Anti-H immunoglobulins can activate the complement cascade, it will lead to the lysis of red blood cells while they are still in the circulation, provoking an acute hemolytic transfusion reaction. This, of course, cannot be prevented