DefinitionABO incompatibility is a reaction of the immune system that occurs if two different and not compatible blood types are mixed together.Causes, incidence, and risk factorsA, B, and O are the three major blood types. The types are based on small substances (molecules) on the surface of the blood cells. In people who have different blood types, these molecules act as immune system triggers (antigens).Each person has a combination of two of these surface molecules. Type O lacks any molecule. The different blood types are:Type A (AA or AO molecules)Type B (BB or BO molecules)Type ABType OPeople who have one blood type form proteins (antibodies) that cause their immune system to react against other blood types. Being exposed to another type of blood can cause a reaction. This is importa