The cellular antigens get attached to the cell surface of the red blood cells. Here we talk about the antigen-antibody interactions. The antigens often get recognized as foreign molecules and trigger the immune response. The antibodies are the immune molecules that recognize and bind to the antigens for processing them further. An individual consists of a large number of cell surface antigens. Hence, the blood group matching plays a very important role, especially during the blood transfusion. Mismatched blood groups trigger serious immune response and also lead to the death of the individual. Hence, a careful blood group matching procedure gets conducted before the blood transfusions. The people with A blood group have A antigen on their cell surface. The people with B blood group have B antigen on their cell surface. The O blood group individuals have none of the above antigens on their cell surface. Only the AB blood group individuals have A and B antigens on their cell surface. Agglutination involves clumping of red blood cells due to the interactions between the antigen and the antibody. The procedure of finding a blood group of an individual is known as blood typing. The antibodies against the antigen A agglutinate only the red blood cells having A antigen on their cell surface. Similarly, the antibodies against the antigen B agglutinate or clump the cells having B antigen on their cell surface. The blood serum prepared from the individuals with A blood group consists of anti-B antibodies and no anti-A antibodies. The blood serum of the B blood group individuals consists of anti-A antibodies. The individuals with AB blood group neither have anti-A nor have anti-B antibodies in their blood serum. The blood serum of the individuals having O blood group consists of both anti-A and anti-B antibodies.