Every blood type can donate to at least one other type, but the rules depend on which antigens sit on your red blood cells and which antibodies float in your plasma. Type O negative can donate red blood cells to anyone, making it the universal red cell donor. Type AB positive can receive red blood cells from anyone, making it the universal recipient. Between those two extremes, compatibility follows a logical pattern once you understand the A, B, and Rh markers.
How Blood Types Are Determined
Your blood type comes from two systems working together. The ABO system sorts you into A, B, AB, or O based on which sugar molecules (antigens) coat the surface of your red blood cells. Type A cells carry A antigens, type B carry B antigens, AB carries both, and O carries neither. Your body naturally produces antibodies against whichever antigens you lack. So type A blood contains anti-B antibodies, type B contains anti-A, type O contains both anti-A and anti-B, and type AB contains neither.
The Rh system adds a second layer. If your cells carry the Rh D antigen, you’re positive. If they don’t, you’re negative. Combining both systems gives the eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−.
The Complete Red Blood Cell Compatibility Chart
When someone needs a red blood cell transfusion, the donor’s antigens must not trigger the recipient’s antibodies. Here’s who can donate to whom:
- O negative can donate to all eight blood types. Because O cells carry no A, B, or Rh antigens, no recipient’s immune system will attack them.
- O positive can donate to A+, B+, AB+, and O+. The Rh antigen limits it to positive recipients only.
- A negative can donate to A+, A−, AB+, and AB−.
- A positive can donate to A+ and AB+.
- B negative can donate to B+, B−, AB+, and AB−.
- B positive can donate to B+ and AB+.
- AB negative can donate to AB+ and AB−.
- AB positive can donate red blood cells only to other AB+ recipients.
The pattern is straightforward: you can donate to anyone who shares your antigens (or has more antigens than you do). You can never donate to someone who would see your antigens as foreign.
Why Mismatched Blood Is Dangerous
If incompatible blood enters your body, your pre-existing antibodies latch onto the foreign antigens coating the donor’s red blood cells. Some antibodies trigger a chain reaction of enzymes called the complement cascade, which punches holes in the cell membranes and destroys the cells outright. Other antibodies cause the donor cells to clump together, a process called agglutination. These clumps are then cleared by immune cells or can block small blood vessels. The result is a hemolytic transfusion reaction, which can cause fever, kidney failure, shock, and in severe cases death.
Why Rh Negative Matters More Than You Think
People with Rh-positive blood can safely receive both Rh-positive and Rh-negative transfusions. But Rh-negative individuals can only receive Rh-negative blood. The stakes go beyond a single transfusion, especially for women who may become pregnant.
If an Rh-negative woman is exposed to Rh-positive blood, whether through a transfusion or during pregnancy with an Rh-positive baby, her immune system can become sensitized. As little as 0.1 mL of Rh-positive blood is enough to trigger this response. The first exposure typically produces antibodies that can’t cross the placenta. But in a subsequent pregnancy with another Rh-positive baby, the immune system rapidly produces a different class of antibody that does cross the placenta, attacking the baby’s red blood cells. This condition, called hemolytic disease of the fetus and newborn, can cause severe anemia and life-threatening complications. It’s one reason hospitals are extremely careful about Rh matching.
Plasma Rules Work in Reverse
Everything flips when you’re talking about plasma instead of red blood cells. With plasma, the concern is the antibodies in the donated liquid, not the antigens on cells. Type AB plasma contains no anti-A or anti-B antibodies, so it’s safe for everyone. That makes AB the universal plasma donor. Type O plasma, on the other hand, contains both anti-A and anti-B antibodies, so it can only go to other type O recipients without risking a reaction.
This reversal matters in emergency rooms. Trauma patients who need both red cells and plasma may receive O-negative red blood cells and AB plasma before their blood type is even known.
O Negative in Emergencies
When someone arrives at a hospital bleeding heavily and there’s no time to determine their blood type, O-negative red blood cells go in first. Many hospitals keep uncrossmatched O-negative units stocked in operating rooms, intensive care units, and trauma bays specifically for this purpose. A study at a major children’s hospital found that 79% of emergency-release uncrossmatched transfusions went to patients with congenital heart disease undergoing procedures or post-operative care, situations where there simply isn’t time for standard testing.
The catch is supply. O negative is relatively uncommon, yet it’s the first blood type to be used in emergencies. This is why blood banks are almost always actively seeking O-negative donors.
Platelets Follow Their Own Rules
Platelet transfusions add another layer of complexity. Platelets carry ABO antigens on their surface (though only 4% to 7% of people express them strongly) and are suspended in donor plasma that contains ABO antibodies. They don’t carry Rh antigens, but they contain trace amounts of red blood cells that can sensitize an Rh-negative recipient to the Rh antigen.
Unlike mismatched red blood cells, out-of-group platelets won’t cause the same explosive hemolytic reaction. The bigger risk comes from the plasma they’re suspended in. If that plasma contains antibodies incompatible with the recipient’s red blood cells, it can cause hemolysis that, while rare, has been fatal. About 19% of platelet transfusions in the United States are plasma-incompatible, which is why transfusion services are required to have specific policies managing this risk.
The Rarest Blood Type of All
Standard blood typing doesn’t catch every variant. The Bombay phenotype, first identified in Mumbai, India, looks like type O on routine testing because it lacks A and B antigens. But it also lacks a foundational antigen called H that all other blood types carry, including type O. People with the Bombay phenotype produce antibodies against H, meaning they will react to transfusions from every standard blood type, including O negative.
The only safe option is blood from another Bombay-type donor. This phenotype occurs in roughly 1 in 10,000 people in parts of India and as rarely as 1 in 1,000,000 in European populations. For these individuals, finding compatible blood in an emergency is a serious logistical challenge, particularly in regions where the phenotype is virtually unheard of. It’s a striking reminder that “universal donor” has limits the standard compatibility chart doesn’t capture.