Blood group systems are classification systems for human blood based on the presence or absence of specific antigens on the surface of Red Blood Cells (RBCs). The two most clinically important systems are the ABO system and the Rh system.
The ABO blood group is controlled by a single gene called the I gene, which has three alleles — making it an example of multiple alleles:
| Allele | Description |
|---|---|
| Codes for Antigen A on RBCs | |
| Codes for Antigen B on RBCs | |
| Recessive; codes for no antigen |
Both and are dominant over , but and are codominant with each other — when both are present, both antigens are expressed.
| Blood Group (Phenotype) | Possible Genotypes | Antigens on RBCs | Antibodies in Plasma |
|---|---|---|---|
| A | or | Antigen A | Anti-B |
| B | or | Antigen B | Anti-A |
| AB | Antigen A and B | None | |
| O | None | Anti-A and Anti-B |
Key Rule: The plasma contains antibodies against whichever antigens are absent from the RBCs.
Blood Group AB is the classic example of codominance in genetics. The genotype results in the expression of both Antigen A and Antigen B on the RBC surface simultaneously. Neither allele is dominant over the other — both are fully expressed. This is distinct from incomplete dominance, where a blended intermediate phenotype results.
Agglutination is the clumping of RBCs that occurs when an antigen on the RBC surface encounters its corresponding antibody in the plasma:
This reaction can lead to hemolysis (destruction of RBCs) and is the basis of blood compatibility testing before transfusion.
| Agglutination with Anti-A | Agglutination with Anti-B | Blood Group |
|---|---|---|
| Yes | No | A |
| No | Yes | B |
| Yes | Yes | AB |
| No | No | O |
| Term | Blood Group | Reason |
|---|---|---|
| Universal Donor | O negative (O−) | RBCs lack A, B, and Rh antigens — no immune response triggered in any recipient |
| Universal Recipient | AB positive (AB+) | Plasma lacks Anti-A, Anti-B, and Anti-Rh antibodies — can receive any blood type |
The Rh system is the second most important blood group system. It is based on the presence or absence of the D-antigen (Rh factor) on RBCs:
Unlike ABO antibodies, anti-Rh antibodies are not naturally present in the plasma. They only develop after exposure to Rh-positive blood.
Also called Haemolytic Disease of the Newborn (HDN), this condition arises when:
Prevention: Rh-negative mothers are given anti-D immunoglobulin (RhoGAM) after delivery to prevent sensitisation.