Blood Types

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.

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.

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.

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.

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.

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.

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.