Why Do We Have Blood Types? A, B, O and the Rhesus Factor Explained
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Before 1901, transfusing blood from one person to another sometimes saved a life and sometimes killed the patient within minutes, and nobody knew why. Karl Landsteiner found the answer by mixing the blood of his laboratory colleagues in test tubes and watching which combinations clumped. People come in a few types, defined by molecules on the surface of their red cells, and the immune system of one type treats the cells of another as invaders.
Sugars on the cell surface
Red blood cells carry various molecules on their outer membrane, and the ABO system concerns one family of them: short chains of sugar. Everyone makes a base chain, called H. People of type A have an enzyme that adds one particular sugar to the end of it; people of type B have a slightly different enzyme that adds a different sugar; people of type AB have both enzymes and make both; people of type O have neither working enzyme and their cells carry the bare H chain. The letters are labels for which decoration the cells wear.
The genetics is simple enough to be a standard school example. A single gene has three common versions, A, B and O, and everyone carries two copies. A and B are dominant over O, so a person with one A and one O copy is type A, and only two O copies give type O. Two type A parents can therefore have a type O child, which has caused arguments in more than one family.
The antibodies that make it dangerous
The reason blood type matters is not the sugar but the immune response to it. From the first months of life, people make antibodies against whichever ABO sugars their own cells lack, apparently because gut bacteria carry similar sugars and the immune system learns to attack the unfamiliar ones. A type A person has antibodies against B; a type B person against A; a type O person against both; a type AB person against neither.
Give a type A patient type B blood and their antibodies bind the transfused cells, which clump and burst, releasing haemoglobin into the bloodstream, clogging the kidneys and setting off a cascade of clotting and shock. It can kill within the hour. That is why type O red cells, which carry no A or B sugar for anyone's antibodies to attack, can be given to anyone in an emergency, and why type AB people, who have no such antibodies, can receive any type. The compatibility rules for red cells:
- •Type O: can donate to anyone, can receive only O
- •Type A: can donate to A and AB, can receive A and O
- •Type B: can donate to B and AB, can receive B and O
- •Type AB: can donate only to AB, can receive from anyone
Positive and negative
The plus or minus after the letter is a separate system. The Rhesus factor, named after the monkeys it was first found in, is a protein on the red cell surface that about 85 percent of Europeans have and 15 percent lack; the proportions differ elsewhere, with almost all East Asians being positive. Unlike the ABO antibodies, antibodies against the Rhesus protein are not made in advance. A negative person makes them only after being exposed to positive blood, which means a first mismatched transfusion may pass but a second will not.
The exposure that matters most is pregnancy. A Rhesus-negative mother carrying a positive baby can be sensitised by the baby's blood at birth, and in a later pregnancy her antibodies can cross the placenta and destroy the next positive baby's red cells, a condition that killed thousands of newborns a year until the 1960s. It is now prevented by giving negative mothers an injection of ready-made anti-Rhesus antibody around the time of birth, which mops up any fetal cells before her own immune system notices them. There are dozens of other blood group systems beyond ABO and Rhesus, but these two account for nearly all transfusion reactions.
Why the types exist
The ABO gene is at least twenty million years old and is shared with other apes, and the fact that the different versions have persisted for so long suggests they are being kept by natural selection rather than drifting. The likeliest explanation is disease. Pathogens use cell-surface sugars as handholds, and different types are vulnerable to different ones. Type O people are less likely to die of severe malaria, which may be why O is so common in the tropics, and more susceptible to cholera and to the stomach ulcer bacterium. Type A carries a higher risk of some cancers and of severe illness from certain viruses.
A population in which several types coexist is harder for any single pathogen to sweep through, which is the same logic that keeps many genes variable. The types are not better or worse. They are a hedge, distributed across the species, against whatever the next epidemic turns out to be. There is no good evidence that blood type affects personality, as a popular Japanese belief holds, or that eating for your blood type does anything.
The takeaway
Blood types are defined by sugars and proteins on the surface of red cells and by the antibodies people carry against the ones they lack, which is why a mismatched transfusion destroys the donated cells. The ABO system depends on which sugar an inherited enzyme adds, the Rhesus factor is a separate protein that matters most in pregnancy, and the variety persists because different types resist different diseases.