ABOallelen

ABOallelen are genetic variants at the ABO gene that determine an individual's ABO blood group antigens on red blood cells and in various tissues. The ABO gene is located on chromosome 9 and encodes glycosyltransferase enzymes that modify a precursor H antigen to produce the A or B antigens. A third allele, commonly called i or O, encodes an inactive enzyme, leaving the H antigen unmodified.

The three major ABOallellen are IA, IB, and i (O). IA and IB are codominant, meaning both can be expressed when present together, while i is recessive. Genotype-to-phenotype relationships are: IAIA or IAi yields the A phenotype; IBIB or IBi yields B; IAIB yields AB; and ii yields O. Suballeles exist within the IA and IB groups (for example A1 and A2) and can influence the amount of antigen expressed.

ABOallellen contribute to transfusion compatibility, organ transplantation considerations, and certain clinical conditions. Mismatches between donor and recipient ABO antigens can trigger acute hemolytic reactions. In transfusion practice, type O blood is often called a universal donor for red blood cells, while type AB blood is a universal donor for plasma; compatibility rules also depend on antibody presence in the recipient. Beyond blood, ABO antigens are expressed on various tissues and secreted in bodily fluids in some individuals, with secretor status influenced by additional genetic factors.

Population genetics show wide variation in ABOallellen frequencies across groups, influencing the distribution of A, B, AB, and O phenotypes globally. Suballeles and rare variants continue to contribute to diversity in ABO antigen expression.