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Blood Types; Transfusion; and Tissue and Organ Transplantation
Antigenicity Causes Immune Reactions of Blood
When blood transfusions from one person to another were first attempted, immediate or delayed agglutination and hemolysis of the red blood cells (RBCs) often occurred, resulting in typical transfusion reactions that frequently led to death. Soon it was discovered that the blood types of different people have different antigenic and immune properties so that antibodies in the plasma of one blood type will react with antigens on the surfaces of the RBCs of another blood type. If proper precautions are taken, one can determine ahead of time whether the antibodies and antigens present in the donor and recipient blood will cause a transfusion reaction.
Multiplicity of Antigens in the Blood Cells
At least 30 commonly occurring antigens and hundreds of other rare antigens, each of which can at times cause antigen-antibody reactions, have been found on the surfaces of the cell membranes of human blood cells. Most of the antigens are weak and therefore are of importance principally for studying the inheritance of genes to establish parentage.
Two particular types of antigens are much more likely than the others to cause blood transfusion reactions. They are the O-A-B system of antigens and the Rh system.
O-A-B Blood Types
A and B Antigens—Agglutinogens
Two antigens—type A and type B—occur on the surfaces of the RBCs in a large proportion of people. It is these antigens (also called agglutinogens because they often cause RBC agglutination) that cause most blood transfusion reactions. Because of how these agglutinogens are inherited, people may have neither of them on their cells, they may have one, or they may have both simultaneously.
Major O-A-B Blood Types
In transfusing blood from one person to another, the blood of donors and recipients is normally classified into four major O-A-B blood types, as shown in Table 36-1 , depending on the presence or absence of the two agglutinogens, the A and B agglutinogens. When neither A nor B agglutinogen is present, the blood is type O . When only type A agglutinogen is present, the blood is type A . When only type B agglutinogen is present, the blood is type B . When both A and B agglutinogens are present, the blood is type AB .
Table 36-1
Blood Types With Their Genotypes and Constituent Agglutinogens and Agglutinins
| Genotypes | Blood Types | Agglutinogens | Agglutinins |
|---|---|---|---|
| OO | O | – | Anti-A and Anti-B |
| OA or AA | A | A | Anti-B |
| OB or BB | B | B | Anti-A |
| AB | AB | A and B | – |
Genetic Determination of the Agglutinogens
The ABO blood group genetic locus has three alleles , which means three different forms of the same gene. These three alleles—I A , I B , and I O —determine the three blood types. We typically call these alleles A, B, and O, but geneticists often represent alleles of a gene by variations of the same symbol. In this case, the common symbol is the letter “I,” which stands for immunoglobulin .
The type O allele is functionless or almost functionless, so it causes no significant type O agglutinogen on the cells. Conversely, the type A and type B alleles do cause strong agglutinogens on the cells. Thus, the O allele is recessive to both the A and B alleles, which show co-dominance .
Because each person has only two sets of chromosomes, only one of these alleles is present on each of the two chromosomes in any individual. However, the presence of three different alleles means that there are six possible combinations of alleles, as shown in Table 36-1 : OO, OA, OB, AA, BB, and AB. These combinations of alleles are known as the genotypes , and each person is one of the six genotypes.
One can also observe from Table 36-1 that a person with genotype OO produces no agglutinogens, and therefore the blood type is O. A person with genotype OA or AA produces type A agglutinogens and therefore has blood type A. Genotypes OB and BB give type B blood, and genotype AB gives type AB blood.
Relative Frequencies of Different Blood Types
The prevalence of the different blood types among one group of persons studied was approximately as follows:
-
O: 47%
-
A: 41%
-
B: 9%
-
AB: 3%
It is obvious from these percentages that the O and A genes occur frequently, whereas the B gene occurs infrequently.
Agglutinins
When type A agglutinogen is not present in a person’s RBCs, antibodies known as anti-A agglutinins develop in the plasma. Also, when type B agglutinogen is not present in the RBCs, antibodies known as anti-B agglutinins develop in the plasma.
Referring once again to Table 36-1 , note that type O blood, although containing no agglutinogens, does contain both anti-A and anti-B agglutinins . Type A blood contains type A agglutinogens and anti-B agglutinins, and type B blood contains type B agglutinogens and anti-A agglutinins. Finally, type AB blood contains both A and B agglutinogens but no agglutinins.
Titer of Agglutinins at Different Ages
Immediately after birth, the quantity of agglutinins in the plasma is almost zero. At 2 to 8 months after birth, an infant begins to produce agglutinins—anti-A agglutinins when type A agglutinogens are not present in the cells, and anti-B agglutinins when type B agglutinogens are not in the cells. Figure 36-1 shows the changing titers of the anti-A and anti-B agglutinins at different ages. A maximum titer is usually reached at 8 to 10 years of age, and this titer gradually declines throughout the remaining years of life.
Origin of Agglutinins in Plasma
The agglutinins are gamma globulins, as are almost all antibodies, and they are produced by the same bone marrow and lymph gland cells that produce antibodies to any other antigens. Most of them are IgM and IgG immunoglobulin molecules.
But why are these agglutinins produced in people who do not have the respective agglutinogens in their RBCs? A possible answer to this question is that small amounts of type A and B antigens enter the body in food, bacteria, and other ways, and these substances initiate the development of the anti-A and anti-B agglutinins.
For example, infusion of group A antigen into a recipient having a non-A blood type causes a typical immune response, with formation of more anti-A agglutinins than ever. Also, the neonate has few, if any, agglutinins, showing that agglutinin formation occurs almost entirely after birth.
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