Blood compatibility follows a set of rules based on your blood type, which is determined by two things: your ABO group (A, B, AB, or O) and your Rh factor (positive or negative). These eight combinations dictate who can safely give blood to whom. The short version: type O negative can donate red blood cells to anyone, and type AB positive can receive red blood cells from anyone. But the full picture is more nuanced than that.
How Blood Types Work
Your red blood cells carry markers on their surface called antigens. If you have type A blood, your cells carry A antigens. Type B carries B antigens. Type AB carries both, and type O carries neither. Your immune system also produces antibodies against the antigens you don’t have. So if you’re type A, your blood contains antibodies that attack B antigens, and vice versa.
This is why matching matters. If someone with type A blood receives type B red blood cells, their anti-B antibodies latch onto those foreign cells and destroy them. This triggers a hemolytic transfusion reaction, which can range from mild fever and chills to life-threatening shock, kidney failure, and uncontrolled bleeding. ABO incompatibility is the most common cause of severe acute transfusion reactions.
Red Blood Cell Compatibility Chart
For red blood cell transfusions, the key rule is: you can’t give someone blood that carries antigens their immune system will attack. Here’s how each type breaks down:
- Type O negative: Can donate red blood cells to anyone. This is the universal donor type because O negative cells carry no A, B, or Rh antigens, so no recipient’s immune system will recognize them as foreign.
- Type O positive: Can donate to any Rh-positive recipient (A+, B+, AB+, O+).
- Type A negative: Can donate to A and AB recipients, both positive and negative.
- Type A positive: Can donate to A+ and AB+.
- Type B negative: Can donate to B and AB recipients, both positive and negative.
- Type B positive: Can donate to B+ and AB+.
- Type AB negative: Can donate to AB negative and AB positive.
- Type AB positive: Can donate red blood cells only to other AB+ individuals. However, AB positive is the universal recipient, meaning people with this type can safely receive red blood cells from every other blood type. Their blood carries both A and B antigens and the Rh factor, so their immune system doesn’t produce antibodies against any of these markers.
The Rh Factor Adds Another Layer
The Rh factor (the “positive” or “negative” part of your blood type) refers to a protein called the D antigen. If your cells carry it, you’re Rh positive. If not, you’re Rh negative. Rh-negative patients should receive Rh-negative blood. Rh-positive patients can safely receive either.
Unlike ABO antibodies, which your body produces naturally, anti-Rh antibodies only develop after exposure to Rh-positive blood. This means the first mismatched transfusion might not cause an obvious reaction, but it sensitizes the immune system. A second exposure can trigger a delayed hemolytic reaction as the now-primed antibodies destroy the transfused cells.
This sensitization is especially important for women who may become pregnant. If an Rh-negative mother carries an Rh-positive baby, her immune system can develop antibodies against the baby’s blood cells. The first pregnancy is typically fine, but in a subsequent pregnancy with another Rh-positive baby, those antibodies can cross the placenta and attack the fetal red blood cells. This condition, called hemolytic disease of the newborn, can cause severe anemia, brain damage, or even death in the fetus without treatment.
Plasma Rules Are the Reverse
Everything flips when it comes to plasma transfusions. With red blood cells, you worry about the antigens on the donated cells. With plasma, you worry about the antibodies in the donated fluid. Type O plasma contains both anti-A and anti-B antibodies, making it the most restrictive to give. Type AB plasma contains neither, making AB the universal plasma donor.
So while type O negative people are the most valuable red blood cell donors, type AB donors are the most valuable plasma donors. Someone with AB blood can only donate red blood cells to other AB recipients, but their plasma is safe for everyone.
Platelets Follow Their Own Rules
Platelet transfusions are more flexible than red blood cell transfusions. Platelets themselves don’t carry ABO antigens in the same way red blood cells do, so a mismatched platelet transfusion won’t cause the same kind of severe destruction. That said, platelet donations come suspended in a small amount of plasma, which does contain antibodies. A minor mismatch (where the donor’s plasma antibodies are incompatible with the recipient’s red blood cells) can occasionally cause some red blood cell damage.
Platelets also don’t carry Rh antigens, but platelet donations can contain trace amounts of red blood cells. This means an Rh-negative person who receives platelets from an Rh-positive donor could become sensitized to the Rh factor, which matters most for women of childbearing age.
Rare Blood Types Complicate Matching
Beyond the standard eight blood types, some people have extremely rare variations that make finding compatible donors difficult. The most well-known is the Bombay phenotype, found in about 1 in 10,000 people in India and 1 in a million in Europe. People with this type lack a foundational antigen called H, which is the building block for A and B antigens. Their red blood cells appear similar to type O, but they produce antibodies against the H antigen that type O blood carries in abundance.
This means a person with the Bombay phenotype cannot receive standard type O blood, even though it would seem like a match. A transfusion of “normal” type O blood can trigger a severe, life-threatening reaction. They can only receive blood from other donors with the same rare phenotype, which often requires maintaining specialized registries and frozen blood reserves.
Who Can Donate Blood
Eligibility to donate goes beyond just having the right blood type. In the United States, donors generally need to be at least 17 years old (or 16 with parental consent), weigh at least 110 pounds, and be in good health. Your blood pressure and temperature are checked at the time of donation.
Hemoglobin levels also matter. Men need a minimum hemoglobin of 13.0 g/dL, while women need at least 12.5 g/dL. Some collection centers can accept women with hemoglobin as low as 12.0 g/dL under specific FDA-approved protocols. These thresholds exist to make sure donating a pint of blood won’t leave you anemic.
Why Compatibility Testing Still Happens
Even when a blood type match looks right on paper, hospitals still crossmatch blood before transfusion. This involves mixing a small sample of the donor’s red blood cells with the recipient’s plasma to check for reactions. Beyond ABO and Rh, there are hundreds of other minor blood group antigens, including the Kidd, Duffy, and Kell systems, that can cause problems. People who receive frequent transfusions are more likely to develop antibodies against these minor antigens over time, making each successive match harder to find.
The severity of a reaction depends on several factors: how much incompatible blood was transfused, the specific antigen involved, and the type and quantity of antibodies the recipient has. Some reactions are immediate and dramatic. Others show up days or weeks later as a slow, smoldering destruction of the transfused cells, causing unexplained fever and a drop in blood counts.