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Blood Types
Human blood is classified into different types based on the presence of antigens on the red blood cell's surface and antibodies in the plasma. Proper…
ABO blood grouping divides human blood into four types based on the presence or absence of two specific antigens, antigen A and antigen B, on the red blood cell surface. These antigens are encoded by three alleles.
The allele immunoglobulin A, or simply allele A, codes for antigen A on the surface of the red blood cell. Similarly, allele B codes for antigen B.
The third allele - recorded as lowercase i and referred to as allele O - does not code for the antigen, meaning the red blood cells have neither antigen A nor antigen B.
Since the antigen's presence dictates the blood type, the alleles A and B are dominant over allele O, which does not produce any antigens.
Humans are diploid organisms, and therefore they receive two copies of the alleles, one from each parent. A person with two copies of allele A or one copy of A and the recessive allele O has type A blood. Similarly, a person with two copies of allele B or one copy of B and the recessive allele O has type-B blood.
Having one copy of allele A and one of allele B results in type AB blood, an example of codominance, where both antigens are expressed on the red blood cell surface. Conversely, a person with both copies of the recessive non-antigen expressing allele will have type O blood.
Consider a male with blood type AB and a female with blood type O. Their progeny may inherit one copy of either allele A or B from the father and a recessive allele from the mother, resulting in offspring with type A or type B blood.
Rhesus factor is another blood typing system, this time based on the Rh antigen also called the D antigen on the red blood cells. The D antigen's presence is indicated with a plus sign, and its absence is marked with a minus sign.
Combining both typing systems, a person can be classified as A, B, AB, or O with positive or negative Rh factors.
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Q1: What are the three alleles that determine ABO blood type?
ABO blood type is determined by three alleles: allele A codes for antigen A on red blood cells, allele B codes for antigen B, and allele O (the recessive allele) does not code for any antigen. Since alleles A and B are dominant over O, they determine whether antigens appear on the cell surface. The combination of two alleles inherited from each parent produces the four blood types.
Q2: How does codominance explain type AB blood?
Type AB blood results from inheriting one copy of allele A and one copy of allele B, demonstrating codominance. In this case, both alleles are fully expressed, meaning both antigen A and antigen B appear on the red blood cell surface simultaneously. This differs from the dominance pattern seen in type A and type B blood, where only one antigen is expressed.
Q3: What blood types can offspring have from an AB father and type O mother?
Offspring from an AB father and type O mother can only have type A or type B blood. The father contributes either allele A or allele B, while the mother contributes the recessive allele O. This produces either AO (type A) or BO (type B) genotypes in their children, never type AB or type O.
Q4: What does the Rh factor indicate in blood typing?
The Rh factor, also called the D antigen, is another blood typing system based on the presence or absence of this antigen on red blood cells. A plus sign indicates the D antigen is present (Rh positive), while a minus sign indicates its absence (Rh negative). Combined with ABO typing, the Rh factor creates eight major blood type classifications.
Q5: How do the MNS blood group antigens differ from ABO antigens?
MNS blood group antigens are encoded by two highly polymorphic genes, glycophorin A and glycophorin B, which can recombine to produce over 40 different antigens. Unlike the ABO system's three alleles, the MNS system involves multiple genes that create greater genetic diversity. Both systems classify blood based on surface antigens, but they operate independently.
Q6: What is the Bombay blood group and how rare is it?
The Bombay blood group is an extremely rare condition where individuals lack the H-antigen on their red blood cells. This occurs in approximately 4 per million people in the human population. Without the H-antigen base, neither antigen A nor antigen B can be added, making this blood group phenotypically similar to type O but genetically distinct.
Q7: Why is proper blood type identification important for transfusions?
Proper blood type identification is essential for successful blood transfusion because mismatched blood types can trigger immune reactions. The International Society of Blood Transfusion has identified 38 human blood types based on surface antigens. Matching donor and recipient blood types prevents agglutination and ensures safe transfusion outcomes for patients.