Blood Type Compatibility Checker

Understanding blood type compatibility

Every year millions of units of blood are transfused in hospitals around the world, and every single transfusion depends on one fundamental question: are the donor and recipient compatible? Get it wrong and the immune system attacks the donated red cells, causing a haemolytic reaction that can be fatal. Get it right and a patient in crisis gets a lifesaving gift. This tool lets you look up the compatibility rules instantly, whether you are curious about your own type, preparing to donate, or simply studying for a biology exam.

Blood typing began in earnest in 1901 when Karl Landsteiner discovered that mixing blood from two different people sometimes caused the red cells to clump together and sometimes did not. He identified the three main ABO groups — A, B and O — and won a Nobel Prize for it. His colleagues later found the AB type, and in 1940 Landsteiner and Alexander Wiener discovered the Rh factor. Together these two systems give us the eight types in everyday use: A+, A-, B+, B-, O+, O-, AB+ and AB-.

What the ABO system means

Your ABO blood type is determined by which antigens sit on the surface of your red blood cells. Antigens are proteins and sugars that the immune system uses to recognise "self" from "foreign". If you have type A blood, your red cells display the A antigen, and your plasma carries anti-B antibodies. If you have type B, the reverse is true. AB individuals carry both antigens and neither antibody. O individuals carry neither antigen but have both anti-A and anti-B antibodies.

This matters for transfusions because antibodies attack foreign antigens. If a type A person receives type B blood, their anti-B antibodies attack the donated cells, causing them to clump (agglutinate) and break apart. The resulting debris can clog kidneys and trigger a cascade of immune responses. This is why strict type-matching is essential.

The Rh factor

The second major piece of the puzzle is the Rh factor, named after the rhesus monkey used in early research. If your blood carries the Rh antigen (specifically the D antigen), you are Rh-positive (+). If it does not, you are Rh-negative (-). Unlike ABO antibodies, most people do not naturally carry anti-Rh antibodies — they only develop them after exposure to Rh-positive blood through a transfusion or, for Rh-negative mothers, through pregnancy with an Rh-positive baby.

This is why an Rh-negative person can receive Rh-positive blood once without immediate disaster, but a second exposure can cause a severe reaction as the immune system has now learned to fight Rh-positive cells. For this reason, hospitals always try to match the Rh factor just as carefully as the ABO type.

How to read the compatibility table

The tool shows two things for your blood type: who you can donate to and who you can receive from. These are not symmetrical — donating and receiving follow different rules.

When donating, the concern is whether the recipient has antibodies that would attack your donated red cells. O- donors are safe for anyone because their cells carry no antigens that can trigger a reaction. O+ donors are safe for any Rh-positive recipient. AB donors can only give to other AB individuals.

When receiving, the concern is whether your antibodies would attack the donated cells. AB+ recipients can accept any blood because they have no ABO antibodies and are Rh-positive. O- recipients can only receive O- blood because any antigen on the donated cells would trigger their antibodies.

O negative: the universal donor

O- is the most in-demand blood type at blood banks worldwide because it can be given to any patient regardless of blood type. This makes it indispensable in emergencies, mass casualty events and neonatal intensive care units, where there is no time to determine the patient's blood type before treatment. O- is also the safest choice for newborn transfusions because their immune systems are not yet fully developed.

Only about 7–8% of most populations have O- blood, which means supply is chronically tight. If you are O-, you are especially encouraged to donate regularly. Your blood can save a life when there are seconds to spare.

AB positive: the universal recipient

AB+ individuals can receive red cells from all eight blood types, which makes them the easiest to treat in a transfusion emergency. However, they can only donate red cells to other AB+ people. This symmetry — the most flexible receivers are the most restricted donors — is one of the neat facts about blood type genetics.

Note that "universal" applies specifically to red cell transfusions. Plasma compatibility works in reverse: AB plasma can be given to anyone (it contains no ABO antibodies), while O plasma can only go to O recipients. Platelets follow slightly different rules again. This tool focuses on whole blood and red cell compatibility, which covers the most common transfusion scenario.

Blood type and pregnancy

Rh incompatibility becomes especially important in pregnancy. If a mother is Rh-negative and carries an Rh-positive baby (inheriting the father's Rh gene), small amounts of fetal blood can enter the mother's circulation during delivery, causing her to develop anti-Rh antibodies. In a subsequent Rh-positive pregnancy, these antibodies can cross the placenta and attack the baby's red cells, causing haemolytic disease of the newborn.

This risk is well managed with a preventive injection of Rh immunoglobulin (Rhogam) given to Rh-negative mothers during and after pregnancy. Knowing your blood type early in pregnancy allows your doctor to take appropriate steps.

Finding out your blood type

If you do not know your blood type, the easiest way to find out is to donate blood — the testing laboratory will type your blood and the blood service will tell you the result. Many GP surgeries will include blood typing in a routine blood panel. Home blood typing kits are also available that use a small lancet and a card with reagent spots; a drop of blood on each spot reveals your ABO and Rh type within minutes, though these kits are not a substitute for laboratory testing when medical decisions are at stake.

Blood type can also sometimes be inferred from genetics if both parents know their types, but inheritance is complex enough that this is not reliable without an actual test.

Why blood type knowledge matters

Knowing your blood type is useful beyond transfusion emergencies. It is valuable information to carry in your wallet if you travel, especially to regions with less-developed health infrastructure. It is important for family planning in the context of Rh incompatibility. Some research has suggested loose associations between blood type and susceptibility to certain diseases, though these links are statistical curiosities rather than clinical tools. And for many people, it is simply an interesting piece of self-knowledge to have.

This tool presents the standard compatibility rules used worldwide. In practice, blood banks perform additional tests including crossmatching — mixing a sample of the donor's cells with the recipient's serum — to catch rare antibodies beyond the ABO and Rh systems. But the eight-type grid you see here is the foundation of safe transfusion medicine everywhere.