Human Blood Groups

Two people's blood is not chemically interchangeable. Mixing incompatible blood types causes agglutination — visible clumping — of red blood cells (erythrocytes), because the surface of every red cell and epithelial cell carries molecules called antigens, and blood plasma carries antibodies that react against 'foreign' antigens it does not recognise as self. Whether a given transfusion is safe or dangerous comes down entirely to which antigens the donor's cells carry and which antibodies the recipient's plasma carries against them.

The ABO system — the antigens, the antibodies, and the broad composition of blood — was first introduced in Class XI; this chapter picks it up specifically as a case study in multiple allelic inheritance. It is also not a uniquely human system: closely related antigens turn up across primates (A-type antigens in chimpanzees and gibbons; A, B and AB antigens in orangutans), in New World monkeys and lemurs (a substance resembling but not identical to human B antigen), and in cats, which have their own three-blood-group genetic system parallel to the human one. In humans, these same antigens are not confined to bl …