Description
ABO blood group
This is the most significant blood group system in transfusion
practice and the only one for which reciprocal antibodies are
consistently present in sera. These antibodies may cause severe
intravascular hemolysis if ABO-incompatible blood is transfused
into such patients.
In normal human blood, a reciprocal relationship exists between
ABO antigens or agglutinogens on the surface of the erythrocyte
and the natural antibodies or isoagglutinins found in
serum. People with type O have neither A nor B antigens on
erythrocytes, although their serum contains both anti-A and
anti-B agglutinins. Those with type A have antigen A on erythrocytes
and anti-B in serum. People with type B have antigen
B in erythrocytes and anti-A in serum. Those with type AB
have both A and B antigens on erythrocytes but no isoagglutinins
in serum. Types A and AB may be subdivided by anti-A1
serum into types A1 as well as A2, A1B, and A2B. A2 antigen
reacts more weakly than A1, but such difference is also qualitative.
Production of ABO antigens is controlled by a series of
allelic genes A1, A2, B, and O (sometimes designated IA, IA
1,
IB, and i; iA
1, iA
2, iB, and i; or A1, A2, aB, and a) A1 is dominant
to A2, and both are dominant to O; no dominance exists
between A1 and B nor between A2 and B.
In the usual typing method, a strong anti-A serum that agglutinates
cells containing A1 or A2 antigen is used; cells agglutinated
by this serum but not by anti-B are of type A but may be of
genotype A1A1, A1A2, A1O, A2A2, or A2O. Cells of people of
type A that are agglutinated by anti-A1 are of type A1 and may
be of genotype A1A1, A1A2, or A1O; type A cells not agglutinated
by anti-A1 are of type A2 and may be of genotype A2A2 or
A2O. Cells agglutinated by anti-B but not anti-A are of type B
and may be of genotype BB or BO. Cells agglutinated by both
anti-A and anti-B are of type AB and can be divided into types
A1B (genotype A1B) and A2B (genotype A2B) by anti-A1. Cells
not agglutinated by either anti-A or anti-B are of type O and
genotype OO. Cells of type O do not simply lack antigenic substance;
most have an antigen called H that is chemically similar
to antigens A and B and is probably the precursor antigen that
is modified under the influence of genes A1, A2, and B into their
corresponding antigens.
The designation “Bombay” phenotype was assigned to those
whose cells lack A, B, and H antigen and whose serum contains
anti-A, anti-B, and anti-H; they are also referred to as having
the “Oh” phenotype. In addition, weak variants of antigen A
have been described with phenotypes designated A3, A4, A5,
Ax, and Az; more rarely, weak variants of B have been found.
The ABO types are of prime importance with respect to blood
transfusion, and maternal-fetal incompatibility is a frequent
cause of fetal death and erythroblastosis fetalis.