Human blood is classified into different types based on the presence of antigens on the red blood cell's surface and antibodies in the plasma. Proper identification of blood type is essential for successful blood transfusion. The International Society of Blood Transfusion has identified 38 human blood types based on the surface antigens on the red blood cells. The most common types are ABO, Rh, and MNS blood types.

ABO blood group

ABO antigens are glycoproteins encoded by genes present on chromosome 9. The red blood cells have a base glycoprotein moiety made up of Fructose, Galactose, and N-acetylglucosamine called the H-antigen. Blood with only H-antigen on the red blood cell surface is classified as blood type-O. The addition of terminal carbohydrate molecule - N-acetylgalactosamine to the H-antigen makes up antigen-A. The presence of a terminal galactose on the H-antigen makes the antigen B. In a rare blood group called Bombay blood group, patients lack H-antigen on red blood cells. Bombay blood group is an extremely rare condition with an occurrence as low as about 4 per million human population.

MNS blood group

MNS blood group antigens are expressed on the surface of red blood cells. These are encoded by two highly polymorphic genes, glycophorin A (GYPA) and glycophorin B (GYPB). The GYPA codes for M or N antigen, whereas GYPB codes for the S or s antigen and the N antigen ('N'). These genes can recombine to produce over 40 different antigens.

Rh blood group

Rh factor is another antigen present on the red blood cell surface. Based on its presence or absence, a person can be classified as either Rh positive or negative. Landsteiner and Wiener coined the term Rh-factor in 1937. Later on, the human alloantibody was renamed anti-D. So far, more than 45 different antigens are identified in the Rh-system. RH blood grouping is most commonly used along with ABO blood grouping.