Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three
alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are
derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell
membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B
antigen; the O allele does not code for any antigen.
While there are many other antigens found on red blood cell membranes, the second most important antigen is
the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an
individual is Rh+; if it is absent, an individual is Rh−. For example, a person with type AB blood with the Rh
antigen is said to be AB+.
These antigens become most important when an individual comes into contact with foreign blood. Because of
the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not
have these antigens on their red blood cells will form antibodies against them. This is inconsequential until
situations such as blood transfusion, organ transplant, or pregnancy occur.
Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies
produced by its Rh− mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to
foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced.
This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed,
any further Rh+ fetuses are at risk.
The Coombs tests provide a method for determining whether a mother has mounted an immune response
again her baby’s blood. The tests are based on whether or not agglutination occurs when Coombs reagent is
added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the
mother. The indirect Coombs test takes the mother’s serum, which contains her antibodies but no red blood
cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is
positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby’s red
blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby’s red blood cells
must have been attacked by its mother’s anti-Rh antibodies.
How might one most practically assess the risk of erythroblastosis fetalis in a pregnant woman?
Section: Biological Sciences
Full Access to 815 Questions