After the discovery of blood types by Karl Landsteiner, blood was labelled A, B, AB and O, according to the presence of different chemicals on the surface of the red blood cells.
In all populations around the world, type O blood is the most common, followed by type A. Relatively few have type B, and even fewer are AB.
Using incompatible types in a transfusion causes fatal clumping of the blood vessels.
Type O individuals are called universal donors, as their blood can be safely transfused into patients of any blood type. Type AB individuals are universal receivers. In the UK population, 45 per cent are type O, 43 per cent type A, 9 per cent type B and 3 per cent are AB.
Rh (or Rhesus) factor determines another important classification system, and refers to the presence, or absence, of a blood protein that is also present in Rhesus monkeys. Most people areRh-positive. When a pregnant woman is Rh-negative and her husband Rh-positive, her baby may inherit Rh-positive blood from its father. If this happens, the mother has to be given antibodies to prevent an immune response to the baby. If it is discovered too late for this preventative procedure, it may be necessary to give the baby an intra-uterine transfusion of Rh-negative blood.