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Blood Types

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ABO blood grouping divides human blood into four types based on the presence or absence of two specific antigens, antigen A and antigen B, on the red blood cell surface. These antigens are encoded by three alleles.

The allele immunoglobulin A, or simply allele A, codes for antigen A on the surface of the red blood cell. Similarly, allele B codes for antigen B.

The third allele - recorded as lowercase i and referred to as allele O - does not code for the antigen, meaning the red blood cells have neither antigen A nor antigen B.

Since the antigen's presence dictates the blood type, the alleles A and B are dominant over allele O, which does not produce any antigens.

Humans are diploid organisms, and therefore they receive two copies of the alleles, one from each parent. A person with two copies of allele A or one copy of A and the recessive allele O has type A blood. Similarly, a person with two copies of allele B or one copy of B and the recessive allele O has type-B blood.

Having one copy of allele A and one of allele B results in type AB blood, an example of codominance, where both antigens are expressed on the red blood cell surface. Conversely, a person with both copies of the recessive non-antigen expressing allele will have type O blood.

Consider a male with blood type AB and a female with blood type O. Their progeny may inherit one copy of either allele A or B from the father and a recessive allele from the mother, resulting in offspring with type A or type B blood.

Rhesus factor is another blood typing system, this time based on the Rh antigen also called the D antigen on the red blood cells. The D antigen's presence is indicated with a plus sign, and its absence is marked with a minus sign.

Combining both typing systems, a person can be classified as A, B, AB, or O with positive or negative Rh factors.

Overview

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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.

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