Simplified view of blood transfusion compatibilities
Research has provided an understanding of the molecular biology of blood that has been very
important in medical progress. The common use of blood transfusions is only one example of
that. In blood transfusions it is necessary that the blood types of donor and recipient be
compatible, and that compatibility is based on antigens and antibodies, i.e. elements of the
immune system. If incompatible blood samples came together in the human body, the result
would be agglutination (sticking together and clumping) of the donor's red blood cells (RBC) in
the recipient's blood stream, with potentially fatal consequences. For our purposes, to illustrate
an application of the immune system, we will consider only the ABO blood group issue in blood
transfusion, as follows.
Among the macromolecules found on the RBC surface is one called antigen A and another called
antigen B, both of which are genetically determined. These antigens are glycoproteins, which
are protein molecules that have short carbohydrate chains (oligosaccharides) attached to them.
The carbohydrate units differ in antigens A and B. To connect this to our genetics material
earlier: the allele IA causes production of antigen A; the allele IB causes production of antigen
B; the allele i causes production of neither (no antigen). Therefore, designation of the blood
types A, B, AB, O depends on which of these two antigens is present on RBC surfaces (see table
below).
Next, among the many antibodies that blood plasma (sometimes called blood serum) contains are
two of special interest here: anti-A, which can specifically bind antigen A, and anti-B, which
can specifically bind antigen B. The table below shows their presence/absence. It's worth
noting that this situation is highly unusual in the sense that anti-B is present in type A blood (and
anti-A in type B blood) even though the person has never been exposed to the antigen. [The
"rule" is that an antibody isn't produced unless the individual has been exposed to the antigen
that provokes production of that antibody.]
The threat in blood transfusion is that the donor's RBCs may be rapidly agglutinated by the
recipient's antibody as the RBCs enter the recipient's bloodstream. So, mixing incompatible
blood types must be avoided. For example, transfusing type A RBC into a type B person will
result in rapid agglutination of the type A (donor) RBCs by the anti-A antibody in the recipient.
Studying the table below will show that type O can be given to any of the four types (hence it's
called the universal donor) since its RBCs bear neither antigen A nor antigen B. Similarly, type
AB can receive any of the four types (hence it's called the universal recipient) since the plasma
of the AB person contains neither anti-A nor anti-B to agglutinate donor RBCs. It's best, of
course, to have donor and recipient of the same blood type (A into A, B into B, etc.), but that's
not always possible if the blood supply at "blood banks" (Red Cross, e.g.) is inadequate.
You may see that there seems to be a possible problem caused by the antibody of the donor.
That's true, but there are ways now to avoid that one. Donor blood may be treated so as to
eliminate that threat, and donor blood may be separated into its components such that only the
RBCs are transfused. For our purposes, remember that the important factor is compatibility of
the donor's RBCs (antigen present/absent) and the recipient's antibody status.
One final point: agglutination of RBC as referred to here is not the same as blood clot
formation. The complex sequence of events that leads to formation of a blood clot is not the
same as what is considered here in blood transfusion.
Blood
type
antigen on
RBC surface
antibody in
blood plasma
A
A
anti-B
B
B
anti-A
AB
A and B
neither
O
neither
anti-A and anti-B