The clumping of blood from mismatched blood types is a problem for a patient because

The ABO blood group classification is based on antigens on red blood cells and antibodies found in blood plasma. Blood type A has antigen A on the red blood cells and anti - B antibodies in the blood. Blood type B has antigen B on the red blood cells and anti - A antibodies in the blood. Blood type AB has both antigen A and B on the red blood cells but has no anti-A nor anti - B antibodies in the blood Blood type O has no antigen on the red blood cells and has both anti - A and anti B antibodies in the blood.