ABO Blood Types

Karl Landsteiner identified two distinct surface antigens on human red blood cells, which he named antigen A and antigen B. Based on which of these a person's red cells carry, he recognised three blood types — A, B, and O (the type carrying neither antigen, historically called the 'universal donor' type) — and in 1902 two of his students, Von De Castelle and Sturli, found the fourth, rarer type, AB (carrying both antigens, the 'universal recipient' type).

In 1925, Bernstein worked out the genetics behind this: ABO blood type is set by a single gene, historically labelled I (for iso-agglutination) or L (after Landsteiner), sitting on chromosome 9, which exists in three allelic forms — IA, IB and IO. Each allele's job is to encode (or fail to encode) an enzyme that finishes building a particular sugar antigen on the red cell surface. The IA allele encodes a transferase enzyme that adds N-acetyl galactosamine to a precursor molecule (the H substance), producing the A antigen. The IB allele encodes a different transferase that instead adds galactose to the same precursor, producing the B antigen. The IO allele encodes no functional terminal transferase at all — it is a functionally 'null' allele — so it adds nothing, and an IOIO individual's red cells display neither antigen. …