A blood group or blood type is based on the presence or absence of
two proteins (A, B) on the surface of red blood cells. Because two proteins
are involved, there are four possible combinations or blood types (ABO groups):
Type A - Only the A protein is present.
• Type B - Only the B
protein is present.
• Type AB - Both proteins are present.
• Type O - Neither protein is present (about 40 percent of the population).
In addition to the A and B proteins, there is another protein involved called
the Rh factor (Rh for Rhesus monkey, where it was first identified). The
Rh factor is either present (+) or absent (-). Therefore, blood types are
described as the type and Rh factor (such as O+ , A+, AB-).
There are three forms of the gene (alleles) that control the ABO blood group,
which are designated as iA, i B, and i. You have two alleles (one from your
mother and one from your father), which are referred to as your genotype.
The inheritance of the alleles is co-dominant, meaning that if the allele
is present, it gets expressed. The following genotypes will yield these
• iAiA or iAi - Both genotypes produce the A protein
• iBiB or iBi - Both genotypes produce the B protein
• iAiB - This genotype produces the A and B protein
• ii - This genotype produces no protein (type O).
So, your blood type does not necessarily tell you exactly which alleles
you have. For example, a person with blood type A could have either two
iA alleles or one iA allele and one i allele. It is possible for two parents
with the same blood type (A or B) to have a child with type O blood. Both
parents would have to have a mixed genotype, such as one i allele together
with either one iA or one iB allele.
Blood types are determined by placing three drops of blood on a glass microscope
slide. To one drop of blood, a drop of antibody solution to protein A (anti-A)
is added. To the second drop, a drop of antibody solution to protein B is
added. To the third drop, a drop of antibody solution to Rh factor (anti-Rh)
is added. The blood drops and antibody drops are mixed and examined for
clumps of red blood cells, and the blood type is determined. Clumps mean
that the particular protein (A, B, Rh) is present. For example, clumps in
anti-A and anti-Rh, but not anti-B, would indicate a person with type A+
Blood types are important for giving blood from one person to another (transfusion).
The blood types must be matched. If not matched properly, the recipient
will form clumps (clots) in response to the donor's blood. The clots will
lead to heart attacks, embolisms and strokes (transfusion reactions). Two
blood types are special:
• Type O- is called the universal donor
because it can be given to anybody; it has no protein to cause clumps.
• Type AB+ is the universal receiver because the recipient has all
of the proteins and so will not form clumps.
Finally, the Rh factor is important for pregnant women. If an Rh+ man and
an Rh- woman have a child, the child can be Rh+ or Rh-, depending upon the
genotype of the father. If the baby is Rh+, this can cause problems. While
in the womb, some blood cells from the baby will cross the placenta into
the mother's blood stream. The mother will make antibodies to the Rh+ cells.
If the woman becomes pregnant again and if the baby is Rh+, the mother's
anti-Rh antibodies will cross into the baby's blood and destroy its red
blood cells, which can kill the baby. If diagnosed early, it is possible
to save a baby under these circumstances by replacing the baby's blood with
transfusions that are free of the Rh antibodies. Also, if this situation
is known, it is possible to treat a Rh- woman with anti-Rh antibodies (RhoGam)
immediately after childbirth to inactivate the baby's Rh+ cells and prevent
the mother from forming anti-Rh antibodies (desensitize her).