The ABO blood group system, discovered by Karl Landsteiner in 1901, is the most important blood classification system for transfusions. Your blood type is determined by which antigens sit on the surface of your red blood cells and which antibodies your plasma contains.
There are four main blood groups based on the presence or absence of two antigens — called A and B — on the surface of red blood cells:
Blood type is controlled by one gene with three possible alleles: IA, IB, and i. Everyone inherits two alleles, one from each parent. IA and IB are codominant (both expressed when present together), while i is recessive to both.
In addition to ABO, a second classification uses the Rhesus antigen (also called antigen D). If the D antigen is present on your red blood cells, you are Rh positive (+); if absent, you are Rh negative (−). This doubles the number of common blood types from four to eight.
The Rh factor is controlled by the RHD gene. Rh positive (Rh+) is dominant over Rh negative (Rh−). Two Rh-positive parents can have an Rh-negative child if both carry one copy of the recessive allele (i.e., both are Rh+ heterozygous).
A Punnett square is a grid used to predict the probability of offspring inheriting particular genotypes. Each parent contributes one allele to each offspring. For a cross between a parent with genotype IAi and a parent with genotype IBi:
| IB | i | |
|---|---|---|
| IA | IAIB (AB) | IAi (A) |
| i | IBi (B) | ii (O) |
Result: 25% AB, 25% A, 25% B, 25% O — all four blood types are possible.
Blood type compatibility is critical for safe transfusions. Transfusing incompatible blood can trigger a severe immune reaction (haemolytic transfusion reaction) that can be fatal. The key principle: the donor's antigens must not clash with the recipient's antibodies.
Rhesus disease occurs when an Rh-negative mother carries an Rh-positive foetus. During birth, foetal blood cells may enter the mother's circulation. Her immune system produces anti-D antibodies. In subsequent pregnancies with an Rh-positive baby, these antibodies can cross the placenta and destroy the baby's red blood cells, causing anaemia and jaundice.
Prevention: Rh-negative mothers are given an Anti-D injection (Rhesus immunoglobulin) at around 28 weeks and after delivery. This destroys any foetal Rh-positive cells before the mother's immune system can respond, preventing sensitisation. This is a routine antenatal procedure in the UK.
In Japan and South Korea, a person's blood type is popularly believed to influence personality — a concept called "blood type theory" (ketsueki-gata). Type A is said to be organised and anxious, Type B outgoing and selfish, Type O confident and insensitive, and Type AB rational and indecisive. There is no scientific evidence supporting any link between blood type and personality. The major western psychological bodies consider it a form of pseudoscience.
Blood typing has been used in forensics since the 1920s. While it cannot identify an individual (since millions share each blood type), it can help exclude suspects. Modern forensic investigations use DNA profiling (STR analysis) which is far more discriminating. However, ABO typing remains useful when DNA is degraded, and it is still routinely used in paternity testing alongside genetic analysis.
The four main blood groups in the ABO system are A, B, AB, and O. Each is further classified as positive (+) or negative (−) based on the Rhesus (Rh) factor, giving eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, O−. In the UK, O+ is the most common and AB− is the rarest.
O negative (O−) is considered the universal donor for red blood cells because it lacks A, B, and Rh antigens. This means it can be transfused to patients of any blood type in emergencies without causing an immune reaction. O− blood is in very high demand and O− donors are especially encouraged to give regularly.
AB positive (AB+) is the universal recipient for red blood cells. People with AB+ blood have both A and B antigens and the Rh factor, so their immune system will not produce antibodies against any ABO or Rh type donor blood. They can safely receive blood from any of the eight major blood types.
No. Two parents who are both blood type O (genotype ii) can only produce children with blood type O, because each parent can only pass on the recessive i allele. No matter how many children they have, all will be blood type O. For a child to have blood type A or B, at least one parent must carry the IA or IB allele respectively.
Rhesus disease (haemolytic disease of the newborn) occurs when an Rh-negative mother carries an Rh-positive baby. If foetal blood enters the mother's circulation (typically during delivery), her immune system may produce anti-D antibodies. In future pregnancies, these antibodies can cross the placenta and attack the baby's red blood cells. It is prevented in the UK by giving Anti-D immunoglobulin injections to Rh-negative pregnant women at around 28 weeks gestation and within 72 hours of delivery.
AB negative (AB−) is the rarest blood type in the UK, found in approximately 1% of the population. Despite being rare, AB− plasma is actually the universal plasma donor (as plasma compatibility is the reverse of red cell compatibility). AB+ is found in about 3% of people, making both AB types relatively uncommon compared to O+ at 35%.
Yes, this is possible and does not indicate non-paternity. For example, two parents who are both blood type A but carry the recessive i allele (genotype IAi) each have a 25% chance of having a child with blood type O (genotype ii). Similarly, an A+ parent and a B+ parent could have an O− child if both parents are heterozygous for both the ABO gene and the Rh gene.