Question
Who can donate blood to whom? If a person with blood group AB+ needs a transfusion in an emergency, which blood groups can donate? Also, why is O- called the universal donor?
Solution — Step by Step
Blood groups are determined by antigens on RBCs and antibodies in plasma. Group A has A antigens + anti-B antibodies. Group B has B antigens + anti-A antibodies. Group AB has both antigens but no antibodies. Group O has no antigens but both antibodies.
The “no antibodies” part is why AB is the universal recipient — there’s nothing to attack incoming donor cells.
Rh factor is a separate antigen. Rh+ means you have it. Rh- means you don’t.
The rule is simple: Rh- blood can go to anyone (Rh+ or Rh-). But Rh- people cannot receive Rh+ blood — their immune system will mount a reaction.
| Recipient | Can Receive From |
|---|---|
| A+ | A+, A-, O+, O- |
| A- | A-, O- |
| B+ | B+, B-, O+, O- |
| B- | B-, O- |
| AB+ | All groups (universal recipient) |
| AB- | A-, B-, AB-, O- |
| O+ | O+, O- |
| O- | O- only |
AB+ can receive from all 8 blood groups — A+, A-, B+, B-, AB+, AB-, O+, O-.
This is because AB+ has no antibodies in plasma (so any ABO group is fine) and since the recipient already has Rh+ antigens, receiving Rh+ blood causes no reaction.
O- RBCs carry no A, B, or Rh antigens. Any recipient’s immune system sees nothing foreign to attack.
So O- blood can safely go into any patient regardless of their blood group. O- is the universal donor.
Why This Works
The entire ABO compatibility system comes down to one rule: the donor’s antigens must not match the recipient’s antibodies. If A antigen meets anti-A antibody, agglutination (clumping) happens — and that’s a transfusion reaction.
Group O has no antigens on its RBCs, so it never triggers the recipient’s antibodies. That’s why O- can go anywhere. AB has no antibodies in its plasma, so it never attacks incoming donor cells — that’s why AB+ can receive from anyone.
The Rh system layers on top. Since anti-Rh antibodies only develop after exposure to Rh+ blood (sensitization), the first transfusion of Rh+ into an Rh- person may not cause a severe reaction — but the second one will. In clinical practice, we never take that risk.
For NEET, remember the two extremes: O- is the universal donor (no antigens to trigger anyone) and AB+ is the universal recipient (no antibodies to attack any donor). These two facts appear almost every year.
Alternative Method — The Antigen-Antibody Shortcut
Instead of memorizing the full table, just check for conflict:
- List the antigens in the donor’s blood.
- Check if the recipient has antibodies against those antigens.
- If yes → incompatible. If no → compatible.
Example: Can B- donate to A+?
- B- has: B antigen, no Rh antigen
- A+ has: anti-B antibody (and anti-A, but we only care about what the donor brings)
- B antigen meets anti-B → agglutination → incompatible
This method is slower for MCQs but bulletproof for tricky questions where the table feels ambiguous.
Common Mistake
Mistake: Students write “O+ is the universal donor” — this is wrong and costs marks directly in NEET.
O+ has no A or B antigens, but it does have the Rh antigen. Giving O+ blood to an Rh- recipient will cause Rh sensitization. So O+ can only donate to Rh+ recipients (A+, B+, AB+, O+) — not everyone.
Only O- is the true universal donor. If the exam says “universal donor,” always write O-.
“O negative — the one who gives without asking anything in return.”
O has nothing (no antigens), so nobody rejects it. AB has everything (both antigens), so they can accept everything. These two are the anchor points — memorise these and derive the rest.