a. Quite often genes at far distant loci must interact to produce a given trait.
Such interactions are quite common and, in fact, no gene ever really acts wholly by
itself. Among the blood groups, the best example of gene interaction involves the
interaction between the H, Le (Lewis), or transferases, and the effects of the Se
(Secretor) gene which give rise to the 3 red cell phenotypes of the Lewis blood group
b. Genes that affect the expression of other genes are described as suppressor
or modifier genes. The exact mechanisms of these gene interactions are not fully
understood. One phenomenon seen in blood group serology that has been explained
by gene interaction, is weakening of the D antigen when the C gene is in the trans
In general, when a gene or set of allelic genes can be shown to segregate
independently of all other genes responsible for other antigens, the set is designated a
system. Most of the time an immunogenetic system may be defined as a series of
alleles which act to produce chemically related but serologically distinct antigens.
Occasionally, the situation is less clear as in the Lewis blood group system where the
Lea antigen is a product of the Le gene but the antigen Leb results from interaction of at
least 4 genes (ABO, Le, Se, H). Except for the Le gene, none of the remaining 3 genes
is considered a member of the Lewis system! Hence, be aware that there are
inconsistencies in the naming of systems.
Section II. THE ABO SYSTEM
a. In 1900, Landsteiner discovered that human red blood cells could be
classified as A,B, or O, according to the presence or absence of highly reactive antigens
on the red blood cell surface. Several years later, an associate of Dr. Landsteiner
discovered a fourth ABO blood group AB. First to be discovered; these antigens remain
first in significance for transfusion practice.
b. Of all red blood cell antigens, the A and B are the only ones for which the
corresponding antibodies consistently and predictably exist in the blood of normal
individuals. Because of this, compatibility of ABO group is the essential foundation on
which all other pre-transfusion testing rests.