|
Blood Type
|
Genotypes
|
Antigens on RBCs
|
Antibodies in Blood
|
Safe Transfusions To
|
Safe Transfusions
From
|
|
A
|
AA, AO
|
A
|
anti-B
|
A, AB
|
A, O
|
|
B
|
BB, BO
|
B
|
anti-A
|
B, AB
|
B, O
|
|
AB
|
AB
|
A,
B
|
-
|
AB
|
A, B, AB, O
|
|
O
|
OO
|
-
|
anti-A, B
|
A, B, AB, O
|
O
|
Molecular
Genetics of the ABO Blood Type
system
The ABO blood type is
produced by a locus on the long arm of Chromosome 9
(9q32.4). There are a variety of alleles (designated
IA and IB)
that generate one of two antigenic proteins, A
or B, respectively, on the surface of Red
Blood Cells (RBCs). A third class of alleles
(designated i) does not generate an antigen. Blood
type is determined by the presence of one or the other,
both, or neither antigen. Genotypes IAIA
and IAi
both produce blood type A, IBIB
and IBi
both produce type B. Alleles IA and
IB
are therefore genetically dominant to i.
Because the heterozygous genotypes IAIB
produces a distinct blood type AB with both antigens,
alleles IA
and IB
are genetically co-dominant. Genotype ii produces
blood type O.
The immune system generate antibodies in the circulating blood, anti-A
and anti-B,
that react with surface antigens of any type(s) not
present. This is a bit counter-intuitive. In the absence
of B or A antigens on the RBCs,
respectively, anti-B or anti-A are
produced, as in blood types A and B,
respectively. In the absence of neither [i.e.,
the presence of both] antigen, no antibodies
are produced, as in type AB. Finally, in the absence
of both [i.e., the presence of neither], both antibodies
are produced, as in type O. This means that a person
does not react to her own blood type antigens,
but does react to foreign blood type antigens.
Blood type testing consists of mixing whole
blood from the person tested with anti-A, anti-B,
or both antibodies: agglutination
indicates blood types B, A, and AB,
respectively, because of the antibody-antigen reaction. Absence
of
agglutination indicates blood
type O, because there are no antigens present to react.
Blood type determines transfusibility of
whole blood between persons, as indicated in the table above. A
person may receive blood safely only from someone with the same
blood type, or Type O, because the latter lack
antigens. Thus, Type O persons are "Universal
Donors" [hence the common order in medical dramas, "Hang
two units of Type O, stat!"]. Type AB
persons are "Universal Recipients",
because they lack antibodies. Although the conditions for
transfusion to a Type O person are the most restrictive,
because Type O is the most common blood type in most
western populations, banked blood and donors are readily
available or recruitable during emergencies. Type B is
more common in Asian populations. Although Type AB
is the rarest blood type, such persons may receive blood from
any source.