ABO Blood Groups Four type of blood group exist in humans: A, B, AB and O. The blood groups refer to antigens on the surface of red blood cells, Blood group A has A antigens, Blood group B has B antigens, Blood group AB has both A&B, Blood group O has no antigens
In their plasma, people with blood group A have anti-B antigens, People with blood group B have anti-A antigens, People with blood group AB have neither anti-A nor anti-B antigens People with blood group O have both anti-A and anti-B antigens
Certain combinations of blood groups are, therefore, incompatible. If, for example, a person with blood group A was given a blood transfusion of blood group B then the anti-B antigens in the individuals blood would bind to the B antigens on the donated blood This would cause clumping or agglutination and potentially fatal blocking of small blood vessels