Blood compatibility follows a set of rules based on two things: your ABO blood group (A, B, AB, or O) and your Rh factor (positive or negative). Together, these create eight common blood types, and each one can safely receive blood from only certain donors. A mismatch triggers your immune system to attack the transfused cells, which can be life-threatening.
How Blood Types Work
Your red blood cells carry markers on their surface called antigens. Type A blood has A antigens, type B has B antigens, type AB has both, and type O has neither. Your body also produces antibodies against whichever antigens you don’t carry. So if you’re type A, your blood contains antibodies that attack B antigens. Type O blood contains antibodies against both A and B.
This is why a transfusion with the wrong blood type is dangerous. If you receive red blood cells carrying antigens your body doesn’t recognize, your antibodies latch onto the foreign cells and destroy them. This immune reaction can cause fever, kidney failure, and in severe cases, death.
The Rh factor adds another layer. If your blood is Rh-positive, the D antigen is present on your red blood cells. If you’re Rh-negative, it’s absent. Someone who is Rh-negative doesn’t naturally carry anti-D antibodies, but if they’re ever exposed to Rh-positive blood through a transfusion or pregnancy, their immune system can start producing them. Once that happens, any future exposure to Rh-positive blood triggers a reaction.
Red Blood Cell Compatibility Chart
The core rule is simple: you can only receive red blood cells that don’t carry antigens your body will attack. Here’s how it breaks down for all eight types:
- O negative: Can receive from O negative only. Can donate red blood cells to all eight types, which is why it’s called the universal donor.
- O positive: Can receive from O negative and O positive. Can donate to O+, A+, B+, and AB+.
- A negative: Can receive from A negative and O negative. Can donate to A−, A+, AB−, and AB+.
- A positive: Can receive from A+, A−, O+, and O−. Can donate to A+ and AB+.
- B negative: Can receive from B negative and O negative. Can donate to B−, B+, AB−, and AB+.
- B positive: Can receive from B+, B−, O+, and O−. Can donate to B+ and AB+.
- AB negative: Can receive from AB−, A−, B−, and O−. Can donate to AB− and AB+.
- AB positive: Can receive red blood cells from all eight types. This makes AB+ the universal recipient. Can donate only to other AB+ individuals.
The pattern behind this list is consistent. Rh-negative blood can go to both Rh-positive and Rh-negative recipients, but Rh-positive blood can only go to Rh-positive recipients. And you can never receive blood carrying A or B antigens if your body makes antibodies against them.
Why O Negative Is the Emergency Default
When someone arrives at a hospital with massive bleeding and there’s no time to test their blood type, doctors use O negative red blood cells. Because O negative cells carry no A, B, or D antigens, they won’t trigger an immune reaction in any recipient regardless of blood type. This buys time while the lab runs a proper crossmatch.
The catch is supply. O negative is relatively uncommon. In the United States, only about 7% of the population has it. In some populations the frequency is even lower: a large study of blood donors in Northern India found Rh-negative donors of all types made up just 4.3% of the donor pool. This scarcity is why blood banks constantly emphasize O negative donations.
AB Positive: The Universal Recipient
On the other end of the spectrum, people with AB positive blood are the luckiest when they need a transfusion. Their blood carries both A and B antigens and the D antigen, so their immune system produces no antibodies against any of these markers. They can safely receive red blood cells from any donor. Only about 3.4% of the U.S. population has this type, making it one of the rarest.
Plasma Rules Are Reversed
Everything above applies to red blood cell transfusions. Plasma, the liquid portion of blood, follows the opposite logic. With plasma, the concern isn’t the antigens on red cells but the antibodies dissolved in the fluid. When you transfuse plasma, you’re giving the recipient someone else’s antibodies.
Type AB plasma contains no anti-A and no anti-B antibodies, so it can be given to anyone. This makes AB the universal plasma donor. Type O plasma, on the other hand, contains antibodies against both A and B antigens, so it can only go to other type O recipients safely. If you’re donating plasma rather than whole blood, AB is the most versatile type, not O.
Platelet Transfusions
Platelets occupy a middle ground. They carry ABO antigens on their surface, and they’re collected in plasma that contains the donor’s ABO antibodies. So ABO compatibility matters in two directions: mismatched platelets may be cleared from the recipient’s bloodstream faster, reducing effectiveness, and incompatible plasma from the donor can attack the recipient’s red blood cells.
Platelets don’t carry Rh antigens themselves, but platelet products contain trace amounts of red blood cells or red cell fragments from the donor. If a Rh-negative person receives platelets from a Rh-positive donor, those fragments can trigger the production of anti-D antibodies. This is a particular concern for women of childbearing age, because anti-D antibodies developed this way can later attack a Rh-positive fetus during pregnancy. When this situation arises, a preventive injection of Rh immune globulin is given to block that immune response.
Rh Factor and Pregnancy
The Rh system has special significance during pregnancy. When a Rh-negative mother carries a Rh-positive baby (inheriting the D antigen from the father), small amounts of fetal blood can mix with the mother’s during delivery. Her immune system recognizes the D antigen as foreign and begins producing antibodies against it.
The first pregnancy is usually unaffected, because the initial antibodies produced are a type that can’t cross the placenta. But in a subsequent pregnancy with another Rh-positive baby, the mother’s immune system has already switched to producing a smaller, more aggressive antibody that does cross the placenta. These antibodies attack the baby’s red blood cells, causing a condition called hemolytic disease of the newborn. This is preventable with an injection given during and after pregnancy to stop the mother’s immune system from forming those antibodies in the first place.
Rare Types Beyond ABO and Rh
The ABO and Rh systems are the most clinically important, but more than 300 other blood group antigens exist. Most rarely cause problems, but some do. The Bombay phenotype is a striking example. People with this extremely rare blood type lack a foundational molecule that the A and B antigens are built on. Their immune system produces antibodies against that molecule, which means they react to every ABO type, including type O. They can only receive blood from other individuals with the Bombay phenotype or use their own stored blood. Transfusing even type O red blood cells to someone with the Bombay phenotype can cause a fatal reaction.
People with rare phenotypes like this often register with specialized rare blood donor programs, and some bank their own blood in advance of planned surgeries. It’s a reminder that while the eight common blood types cover the vast majority of transfusion needs, compatibility can be more complex than the standard chart suggests.