Blood Type Compatibility — ABO and Rh Factor Explained
Blood type compatibility is critical in medicine — transfusing incompatible blood can trigger a fatal immune response. The ABO and Rh systems determine which blood types can safely receive blood from which donors. This guide covers the genetics of blood type, the compatibility rules, and the science behind why mismatches are dangerous.
Blood Type Compatibility Table
| Blood type | Can donate to | Can receive from | Antibodies in plasma |
|---|---|---|---|
| A+ | A+, AB+ | A+, A−, O+, O− | Anti-B |
| A− | A+, A−, AB+, AB− | A−, O− | Anti-B, Anti-Rh |
| B+ | B+, AB+ | B+, B−, O+, O− | Anti-A |
| B− | B+, B−, AB+, AB− | B−, O− | Anti-A, Anti-Rh |
| AB+ | AB+ only | All blood types | None |
| AB− | AB+, AB− | All Rh-negative types | Anti-Rh |
| O+ | A+, B+, AB+, O+ | O+, O− | Anti-A, Anti-B |
| O− | All blood types | O− only | Anti-A, Anti-B, Anti-Rh |
How to Use the Blood Type Compatibility Tool
- Open the Blood Type Compatibility tool
- Select your blood type (or a donor's blood type) from the dropdown
- Click Check Compatibility to see which blood types can donate to and receive from this type
- Compatible donor types are shown in green; incompatible in red
- Toggle between Transfusion and Organ Donation modes for different compatibility rules
The ABO Blood Group System
The ABO system is controlled by a single gene with three alleles: I^A, I^B, and i (lowercase = recessive).
| Blood type | Genotype options | Antigens on red cells |
|---|---|---|
| A | I^A I^A or I^A i | A antigen |
| B | I^B I^B or I^B i | B antigen |
| AB | I^A I^B (only) | A and B antigens |
| O | ii (only) | Neither A nor B |
The immune system produces antibodies against ABO antigens not present on its own red cells. Type A blood carries anti-B antibodies; Type B carries anti-A; Type O carries both; Type AB carries neither. Transfusing blood with antigens the recipient's immune system targets triggers agglutination (clumping) and haemolysis.
The Rh Blood Group System
The Rh system is named after the Rhesus macaque monkey, in whose blood the antigen was first discovered. The clinically most important Rh antigen is RhD. Rh+ individuals have the D antigen on their red cells; Rh− individuals do not.
Unlike ABO, Rh− individuals do not naturally have anti-D antibodies — they only develop them after exposure to Rh+ blood (via transfusion or pregnancy). This is why the first Rh-incompatible transfusion may seem uneventful, but a second exposure can trigger a severe haemolytic reaction.
Rh Incompatibility in Pregnancy
If an Rh− mother carries an Rh+ fetus (inherited from an Rh+ father), fetal red cells can enter the mother's bloodstream during delivery, causing her to produce anti-D antibodies. In a second Rh+ pregnancy, these antibodies cross the placenta and attack fetal red cells — causing haemolytic disease of the newborn (HDN), which can be severe or fatal.
Prevention: Rh− mothers are given Rh immunoglobulin (RhoGAM) injections during pregnancy and after delivery to neutralise any fetal Rh+ cells before the mother's immune system can mount a response. This prevents sensitisation entirely and has made HDN from Rh incompatibility rare in countries with routine prenatal care.
Common Questions
Why is O− the universal donor for red blood cells?
O− red cells have no A, B, or Rh antigens. Since the recipient's immune system cannot react against antigens that aren't there, O− blood can be transfused into recipients of any ABO and Rh type without triggering an immune response. In emergencies where blood typing is not possible, O− packed red cells are used.
Why is AB+ the universal recipient?
AB+ individuals carry no antibodies against A, B, or Rh antigens (they have all three on their own cells). They can receive red blood cells from any blood type without immune rejection. However, AB plasma is the universal donor for plasma transfusions — the reverse logic applies.
Can two O parents have an AB child?
No. Two parents both homozygous recessive (ii) can only produce ii offspring (blood type O). An AB child (I^A I^B) requires at least one I^A and one I^B allele from parents. If a claimed O × O cross produced an AB child, there is either a lab error, a very rare genetic anomaly (Bombay phenotype), or paternity is not as assumed.
Check Blood Type Compatibility
Select any blood type and instantly see compatible donors and recipients in the Blood Type Compatibility tool — ABO and Rh systems with full transfusion reference.
Open Blood Type Compatibility