The ABO blood group system consists of three allelic genes: the A, B and O.

In the ABO system, the structure of the three antigens differs due to the addition of the following carbohydrate groups to the L-fucose of the H substance:

• Blood group A antigens have an N-acetyl galactosamine
• Blood group B antigens have a D-galactose
• Blood group O antigens are unaltered.

There are six possible genotypes (OO, AA, AO, BB, BO, AB), but the absence of a specific anti-O allows the serological recognition of only four phenotypes (O, A, B, AB). In the United Kingdom blood group O is the commonest and AB the rarest.

The frequency of the blood groups is: O (46%); A (42%); B (9%); and AB (3%).

The A, B and H antigens are present in most body cells including white cells and platelets. In 80% of the population that have secretor genes, these antigens are also found in soluble form in body fluids and secretions (for example, saliva, sweat, plasma and semen).

Anti-A and anti-B are naturally occurring antibodies and are usually IgM. They react optimally at cold temperatures (4°C) and, although reactive at body temperature, they are called cold antibodies.

Immune antibodies like the rhesus antibody, anti-D are commonly IgG. They react optimally at 37°C and are called warm antibodies.

The rhesus system is coded by allelic genes at three closely linked loci.

In Caucasians the commonest rhesus genotype is CDe/cde (32% of people; CDe/CDe is found in 17% of patients). Anti-D is responsible for most of the clinical problems associated with this system.

Anti-C, anti-c, anti-E and anti-e are occasionally seen and can all cause transfusion reactions and the haemolytic disease of the newborn.