Download 0-AB system of antigens

0-A-B system of antigens
Two different but related antigens – type A and type B – occur on the surfaces of the
erythrocytes in different persons. Because the type A and type B antigens in the cells make
the cells susceptible to agglutination, these antigens are called agglutinogens. Blood of donors
and recipients are normally classified into four major groups, depending on the presence or
absence of the two agglutinogens.
Type
0
A
B
AB
Percent
47
41
9
3
Genetic determination of the agglutiongens:
Genes on two adjacent chromosomes, one gene on each chromosome, determine the 0-A-B
blood groups. These are allelomorphic genes that can be any of the three different types, but
only one type on each chromosome: type 0, type A, type B.
There is no dominance among the three different allelomorphs. The type 0 gene is either
functionless or almost functionless, so that it causes either no agglutinogen in the cells or such
a weak agglutinogen that it is normally insignificant. The type A and type B genes cause
strong agglutinogens in the cells. Because there are genes on two chromosomes there are six
possible combinations of genes:
They are 00, 0A, AA, 0B, BB, or AB. These different combinations of genes are known as the
genotypes.
When type A agglutinogen is not present on the cell surface of a person ´s red blood cells,
antibodies known as “anti-A” agglutinins develop in his plasma. When type B agglutinogen is
not present on the cell surface of the red blood cells, antibodies known as “anti-B” agglutinins
develop in the plasma.
Titer of the agglutinins at different ages:
Immediately after birth the quantity of agglutinins in the plasma is almost zero. Two to eight
months after birth, the infant begins to produce agglutinins.
Origin of the agglutinins in the plasma:
The agglutinins are gamma globulins (all antibodies), and are produced by the same cells that
produce antibodies to any other antigens. Most of them are IgG and IgM immunoglobulin
molecules. It is difficult to understand how agglutinins are produced in individuals, who do
not have the respective antigenic substances. Small amounts of group A and B antigens enter
the body in the food, in bacteria, and these substances presumably initiate the development of
the anti-A and anti-B agglutinins.
Rh-system
In comparison with the 0-A-B system, there is one major difference:
In the 0-A-B system agglutinins develop spontaneously, while in the Rh-system spontaneous
agglutinins almost never occur. A person must be massively exposed to an Rh antigen, before
enough agglutinins are produced to cause a significant transfusion reaction.
Rh antigens:
There are six common types of Rh antigens, each of which is called an Rh factor. These types
are designated C, D, E, c, d, and e. A person who has a C antigen will not have the c antigen
(+ the other way around-for each antigen). Also because of the manner of inheritance of these
factors, each person will have one of each of the three pairs of antigens. Only the C, D, and E
antigens are usually antigenic enough to cause significant development of anti-Rh antibodies
(cause transfusion reactions). Anyone who has any of these three antigens, or any
combination of them, is mentioned to be Rh positive. A person who has no C, D, or E
antigens but instead only c, d, and e antigens is mentioned as Rh negative.
Inheritance of Rh factors:
There are three separate loci on one pair of chromosomes, respectively for the three different
pairs of Rh factors. The Rh positive factors are mendelian dominant, so that if either one of
the chromosomes contains a gene for an Rh positive factor, this factor will be present in the
blood. Approximately 85 % of all Caucasoids are Rh positive and 15 % Rh negative. In
American Blacks the percentage of Rh positive is about 95 %.
Rh immune response:
When erythrocytes containing one or more Rh positive factors are injected into an Rh
negative person, anti-Rh agglutinins will develop very slowly. The maximum concentration of
anti-Rh agglutinins will occur approximately two to four months later. On multiple exposure,
to the Rh factor, the Rh negative person eventually becomes strongly sensitized to the Rh
factor.
Characteristics of Rh transfusion reactions:
If an Rh negative person has never been exposed before to Rh positive blood, transfusion of
Rh positive blood causes no immediate reaction at all. In some of these persons anti-Rh
antibodies develop in sufficient quantities during the next two to four weeks to cause
agglutination of the transfused cells that are still in the blood. These cells are than hemolysed
by the reticuloendothelial system.
A delayed transfusion reaction occurs usually mild.
Transfusion of Rh positive blood into the same person, who is now immunized against the Rh
factor will cause an transfusion reaction, which is greatly enhanced and can be as severe as
the reactions with A and B blood.