Body Fluids and Circulation
Blood groups
Decide who can donate blood to whom.
Blood from different people does not always mix safely. The ABO blood groups depend on two possible antigens, A and B, on the surface of the red blood cells. The plasma naturally carries antibodies against whichever antigen the person does not have.
| Blood group | Antigen on RBCs | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
Before a blood transfusion, the donor's blood must be matched with the receiver's. If the receiver's antibodies meet antigens they react against, the red cells clump together, which can be dangerous.
- Group O has no antigens, so its red cells are not attacked by anyone's antibodies: O can give blood to everyone. O is the universal donor, the generous one.
- Group AB has no antibodies, so it does not attack anyone's red cells: AB can receive blood from everyone. AB is the universal recipient.
There is a second antigen to match: the Rh antigen (first found in Rhesus monkeys). About 80% of people have it (Rh positive, Rh+ve); the rest are Rh negative. An Rh-ve person who receives Rh+ve blood starts making anti-Rh antibodies, so Rh must also be matched.
This matters in pregnancy. If an Rh-ve mother carries an Rh+ve baby, a little of the baby's blood may mix with hers at the first delivery, and her body learns to make anti-Rh antibodies. In a later pregnancy with another Rh+ve baby, these antibodies can cross into the baby and destroy its red blood cells, a serious condition called erythroblastosis foetalis. It can be prevented by giving the mother anti-Rh antibodies right after the first delivery.
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