Although all blood is made of the same basic elements, not all blood is alike. In fact, there are eight different common blood types, which are determined by the presence or absence of certain antigens – substances that can trigger an immune response if they are foreign to the body. Since some antigens can trigger a patient's immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching.
The ABO Blood Group System
There are four major blood groups determined by the presence or absence of two antigens – A and B – on the surface of red blood cells:
Group A – has only the A antigen on red cells (and B antibody in the plasma)
Group B – has only the B antigen on red cells (and A antibody in the plasma)
Group AB – has both A and B antigens on red cells (but neither A nor B antibody in the plasma)
Group O – has neither A nor B antigens on red cells (but both A and B antibody are in the plasma).
There are very specific ways in which blood types must be matched for a safe transfusion. See the chart below:
The universal red cell donor has Type O negative blood type.
The universal plasma donor has Type AB positive blood type.
Note: Research carried out showed that the ABO blood types are inherited. We now know that they are determined by genes on chromosome 9, and they do not change as a result of environmental influences during life. An individual's ABO type results from the inheritance of 1 of 3 alleles (A, B, or O) from each parent.
Both A and B alleles are dominant over O. As a result, individuals who have an AO genotype will have an A phenotype. People who are type O have OO genotypes. In other words, they inherited a recessive O allele from both parents. The A and B alleles are codominant. Therefore, if an A is inherited from one parent and a B from the other, the phenotype will be AB. Agglutination tests will show that these individuals have the characteristics of both type A and type B blood.
CAUTION: the inheritance of ABO blood types does not always follow such straightforward rules of inheritance. If you wish to explore the reason why this is true, select the Bombay Phenotype button below.
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