The ABO blood group

ABO phenotypes

The four basic ABO phenotypes are O, A, B, and AB. After it was found that blood
group A RBCs reacted differently to a particular antibody (later called
anti-A1), the blood group was divided into two phenotypes, A1 and
A2. RBCs with the A1 phenotype react with anti-A1 and
make up about 80% of blood type A. RBCs with the A2 phenotype do not
react with anti-A1 and they make up about 20% of blood type A. A1 red
cells express about 5 times more A antigen than A2 red cells, but
both types of red cell react with anti-A, and as far as transfusion purposes are
concerned, the A1 and A2 blood groups are interchangeable.

There are many other subgroups of blood group A in which RBCs tend to weakly
express the A antigen, whereas weak variants of the blood group B phenotype are
rare (2).

The immune system forms antibodies against whichever ABO blood group antigens are
not found on the individual’s RBCs. Thus, a group A
individual will have anti-B antibodies and a group B individual will have anti-A
antibodies. Blood group O is common, and individuals with this blood type will
have both anti-A and anti-B in their serum. Blood group AB is the least common,
and these individuals will have neither anti-A nor anti-B in their serum.

ABO antibodies in the serum are formed naturally. Their production is stimulated
when the immune system encounters the “missing” ABO blood group antigens in
foods or in micro-organisms. This happens at an early age because sugars that
are identical to, or very similar to, the ABO blood group antigens are found
throughout nature.

The ABO locus has three main alleleic forms: A, B, and O. The A allele encodes a
glycosyltransferase that produces the A antigen (N-acetylgalactosamine is its
immunodominant sugar), and the B allele encodes a glycosyltransferase that
creates the B antigen (D-galactose is its immunodominant sugar).

See the structures of the A, B, and O antigens in Stryer’s
Biochemistry

The O allele encodes an enzyme with no function, and therefore neither A or B
antigen is produced, leaving the underlying precursor (the H antigen) unchanged.
These antigens are incorporated into one of four types of oligosaccharide chain,
type 2 being the most common in the antigen-carrying molecules in RBC membranes.
Some of the other enzymes involved in the earlier stages of ABO antigen
synthesis are also involved in producing antigens of the Hh blood group and the
Lewis blood group.