Blood Groups: ABO and Rh Systems
Every red blood cell carries identity markers called antigens on its surface. Your immune system uses these markers to tell "self" from "foreign." If a foreign antigen shows up in your bloodstream, your plasma antibodies attack it. Blood grouping is simply a classification of which antigens a person's red cells carry, and the two systems that matter most clinically are the ABO system and the Rh system.
The ABO System
Two antigens define this system: A and B. Depending on which ones a person inherits, they fall into one of four groups.
| Blood Group | Antigen on RBCs | Antibody in Plasma |
|---|
| A | A | Anti-B |
| B | B | Anti-A |
| AB | A and B | None |
| O | None | Anti-A and Anti-B |
The governing rule is that a person never carries an antibody against their own antigen — that would mean the immune system attacking its own blood cells. So a person with antigen A makes anti-B antibodies (ready to attack any B cells that enter), and a person with no antigens at all (group O) makes both anti-A and anti-B.
Golden rule of transfusion: a recipient can only accept red cells that do not carry an antigen the recipient's own antibodies would attack.
Working Out Transfusion Compatibility
- Group A (anti-B present) cannot accept B or AB blood, but can accept A or O.
- Group B (anti-A present) cannot accept A or AB blood, but can accept B or O.
- Group AB has no antibodies at all, so it can accept blood from any ABO group — the universal recipient.
- Group O carries both antibodies, so it can only accept O blood — but because its red cells carry no antigen for anyone else's antibodies to attack, it can donate to any group — the universal donor.
Picture the recipient's antibodies as locks and the donor's antigens as keys. If a donor key fits a recipient lock, the transfusion is rejected.
The Rh System
The Rh system centres on a single important antigen, the Rh (D) factor. A person is either:
- Rh-positive (Rh+) — carries the Rh antigen on the red cells, or
- Rh-negative (Rh-) — does not carry it.
Unlike the ABO system, an Rh-negative person is not born with anti-Rh antibodies. Those antibodies only appear after the person's immune system has actually been exposed to Rh-positive blood, through a transfusion or a pregnancy.
- An Rh-negative person should receive Rh-negative blood.
- An Rh-positive person can safely receive either Rh-positive or Rh-negative blood.
The real danger is sensitisation: once an Rh-negative person has built anti-Rh antibodies from a first exposure, a later transfusion (or pregnancy) involving Rh-positive blood can trigger a severe immune reaction.
Combining Both Systems …