The four blood groups are A, B, AB, and O. These groups are determined by the ABO system, which classifies blood based on specific molecules (called antigens) found on the surface of your red blood cells. Your blood group matters most during transfusions and pregnancy, where a mismatch between blood types can trigger a dangerous immune reaction.
How the Four Groups Differ
What separates one blood group from another comes down to two things: the antigens on your red blood cells and the antibodies floating in your plasma. Your body naturally produces antibodies against whichever antigens your own blood cells lack. This is why mixing the wrong blood types causes problems.
- Type A: Has A antigens on red blood cells and anti-B antibodies in the plasma.
- Type B: Has B antigens on red blood cells and anti-A antibodies in the plasma.
- Type AB: Has both A and B antigens on red blood cells and no ABO antibodies in the plasma.
- Type O: Has no A or B antigens on red blood cells but carries both anti-A and anti-B antibodies in the plasma.
This pattern explains why type O blood can be given to almost anyone (there are no antigens for a recipient’s immune system to attack), while type AB recipients can accept blood from any group (they have no antibodies to reject incoming cells).
The Rh Factor: Positive and Negative
Each of the four blood groups is further split by the Rh factor, a separate protein on the surface of red blood cells. If you have this protein, you’re Rh-positive (+). If you don’t, you’re Rh-negative (-). This creates eight common blood types in total: A+, A-, B+, B-, AB+, AB-, O+, and O-.
O+ is the most common blood type worldwide, found in roughly 40% of the population. AB- is the rarest, occurring in less than 1% of people globally. The Rh factor is especially important during pregnancy: if a mother is Rh-negative and her baby is Rh-positive, her immune system can develop antibodies against the baby’s blood cells, a condition that’s preventable with treatment but requires early detection.
How You Inherit Your Blood Type
Your blood group is determined by a single gene you inherit from each parent. You get one copy from your mother and one from your father, and the combination determines your type. The A and B versions of the gene are co-dominant, meaning if you inherit one of each, both are expressed and you end up with type AB. The O version is recessive, so it only shows up when you inherit it from both parents.
This is why two parents who are both type A can have a child with type O. Each parent might carry one A gene and one hidden O gene (genotype AO). If both pass along their O gene, the child will be type O. Similarly, a parent with type B blood could carry a BO genotype. Two parents with types A and B could potentially have children with any of the four blood groups, depending on their exact genetic makeup.
Who Can Donate to Whom
Blood type compatibility follows strict rules. If you receive blood with antigens your body doesn’t recognize, your immune system treats those donor cells as invaders and destroys them. This is called a hemolytic transfusion reaction, and in its acute form it can cause fever, chills, chest and back pain, confusion, and dizziness within minutes. Delayed reactions can develop days later and cause fatigue and yellowing of the skin and eyes.
Here’s how compatibility works for red blood cell transfusions:
- O- can donate to anyone and is used in emergencies when a patient’s blood type is unknown. O+ can donate to any positive type.
- A+ can receive from A+, A-, O+, and O-. A- can receive from A- and O- only.
- B+ can receive from B+, B-, O+, and O-. B- can receive from B- and O- only.
- AB+ is the universal recipient, compatible with all eight types. AB- can receive from any negative type.
- O+ can only receive from O+ and O-. O- can only receive from O-.
Notice the pattern: Rh-negative recipients can only receive Rh-negative blood, while Rh-positive recipients can accept either. And you can never receive blood with an A or B antigen that your body carries antibodies against.
How Blood Typing Works
Determining your blood type involves two complementary tests. In forward grouping, a lab mixes a sample of your red blood cells with known antibodies (anti-A, anti-B, and anti-Rh). If the cells clump together when mixed with anti-A antibodies, your blood carries the A antigen. In reverse grouping, your plasma is mixed with known type A and type B red blood cells to confirm which antibodies you carry. The two tests should agree, and when they do, your blood type is confirmed.
Most people learn their blood type through a routine blood draw, a blood donation, or prenatal testing. Home blood typing kits also exist, though they’re less reliable than lab testing.
Beyond the Standard Four Groups
The ABO system is the most clinically important blood classification, but it isn’t the only one. Scientists have identified more than 40 blood group systems involving hundreds of different antigens. Most of these rarely cause problems in transfusions, which is why the ABO and Rh systems get the most attention.
One notable exception is the Bombay phenotype (also called Oh), a rare condition in which a person lacks a foundational molecule called H substance that serves as the building block for A and B antigens. People with Bombay blood test as type O but can only receive blood from other Bombay donors, not from standard type O. It occurs in about 1 in 10,000 people in parts of India and roughly 1 in a million in Europe, making compatible donors extremely scarce.