Group O is the universal red cell donor; group AB the universal plasma donor
Mnemonic
“O is the universal donor, AB the universal recipient”, which follows from the antigens and antibodies:
| Group | Antigen on cell | Antibody in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
“You make antibodies against what you do not have.”
The rule inverts for plasma and platelets: since the danger there is the donor’s antibodies rather than antigens, AB is the universal plasma donor and O the universal plasma recipient.
RhD matters chiefly in pregnancy: an RhD negative mother carrying an RhD positive fetus can be sensitised, causing haemolytic disease of the newborn in later pregnancies, which is why anti-D prophylaxis is given after any sensitising event.
ABO antibodies are IgM and naturally occurring, so ABO incompatible transfusion causes immediate intravascular haemolysis; anti-D is IgG, crosses the placenta, and causes delayed extravascular haemolysis.
Expansion
| Group | Red cell antigen | Plasma antibody | Can receive red cells from |
|---|---|---|---|
| A | A | anti-B | A, O |
| B | B | anti-A | B, O |
| AB | A and B | none | All (universal recipient) |
| O | none | anti-A and anti-B | O only (universal donor) |
For plasma the logic reverses: AB plasma contains no antibodies and is the universal plasma donor.
ABO antibodies are naturally occurring IgM, formed in infancy against gut bacterial antigens, and cause immediate intravascular haemolysis if incompatible blood is given.
Rhesus D antibodies are not naturally occurring; they form only after exposure through transfusion or pregnancy, are IgG and therefore cross the placenta.
This is why a RhD negative mother carrying a RhD positive fetus can become sensitised and, in a subsequent pregnancy, cause haemolytic disease of the newborn. Prophylactic anti-D given at sensitising events and routinely in the third trimester prevents sensitisation by clearing fetal cells before the maternal immune system responds.