The four blood types are A, B, AB, and O. They’re determined by the ABO blood group system, which classifies blood based on specific markers (called antigens) found on the surface of red blood cells. When you factor in the Rh system (positive or negative), these four types expand into eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
What Makes Each Blood Type Different
Your blood type comes down to which sugar molecules sit on the surface of your red blood cells. These molecules act like identity tags. Type A blood carries the A antigen. Type B carries the B antigen. Type AB carries both. Type O carries neither.
Your immune system also produces proteins called antibodies that react against the antigens you don’t have. This is where transfusion safety becomes critical:
- Type A: Has A antigens on red blood cells and produces anti-B antibodies
- Type B: Has B antigens on red blood cells and produces anti-A antibodies
- Type AB: Has both A and B antigens and produces no ABO antibodies
- Type O: Has no A or B antigens and produces both anti-A and anti-B antibodies
If you receive blood with antigens your body doesn’t recognize, those antibodies attack the foreign red blood cells. This is why matching blood types before a transfusion is essential.
The Rh Factor: Positive vs. Negative
Beyond the ABO system, there’s a second classification layer called the Rh system. It hinges on one specific antigen, the D antigen. If your red blood cells carry it, you’re Rh-positive. If they don’t, you’re Rh-negative. Combining ABO type with Rh status gives you the eight common blood types most people are familiar with.
Rh status matters most during pregnancy. When an Rh-negative mother carries an Rh-positive baby, her immune system can treat the baby’s blood cells as foreign and build antibodies against them. Before preventive treatment was introduced in the 1970s, this caused serious harm: 14% of affected babies were stillborn, and half of those born alive suffered death or brain injury. A targeted injection given during and after pregnancy now reduces that risk to roughly 0.1 to 0.2%.
How Blood Type Is Inherited
You inherit one ABO gene from each biological parent. Three versions of this gene exist: A, B, and O. The A and B versions are co-dominant, meaning if you inherit one of each, both get 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 creates some combinations that surprise people. Two parents with type A blood can have a child with type O, because each parent might carry a hidden O gene alongside their A gene. Similarly, a parent with type A and a parent with type B could produce a child with any of the four blood types, depending on which genes each parent passes along. Rh status follows its own inheritance pattern, with positive being dominant over negative.
Who Can Donate to Whom
Blood type compatibility follows a logical pattern based on antigens and antibodies. Two types get special designations. Type O negative is the universal red blood cell donor because its cells carry no A, B, or Rh antigens, so no recipient’s immune system will attack them. Type AB positive is the universal recipient because it produces no ABO antibodies and is Rh-positive, meaning it can safely accept red blood cells from any type.
Here’s how the eight types break down for red blood cell transfusions:
- O-: Can donate to all types, can receive only from O-
- O+: Can donate to any positive type (O+, A+, B+, AB+), can receive from O+ and O-
- A-: Can donate to A+, A-, AB+, AB-, can receive from A- and O-
- A+: Can donate to A+ and AB+, can receive from A+, A-, O+, O-
- B-: Can donate to B+, B-, AB+, AB-, can receive from B- and O-
- B+: Can donate to B+ and AB+, can receive from B+, B-, O+, O-
- AB-: Can donate to AB+ and AB-, can receive from AB-, A-, B-, O-
- AB+: Can donate only to AB+, can receive from all types
How Common Each Type Is
Blood type distribution varies by ethnicity and geography, but data from the U.S. population gives a useful snapshot. The two most common types together account for nearly three-quarters of the population:
- O+: 37.4% (about 1 in 3 people)
- A+: 35.7% (about 1 in 3)
- B+: 8.5% (about 1 in 12)
- O-: 6.6% (about 1 in 15)
- A-: 6.3% (about 1 in 16)
- AB+: 3.4% (about 1 in 29)
- B-: 1.5% (about 1 in 67)
- AB-: 0.6% (about 1 in 167)
AB negative is the rarest of the standard eight types. O negative, despite being the universal donor, makes up less than 7% of the population, which is why blood banks are constantly appealing for O-negative donations.
Beyond the Standard Four
The ABO and Rh systems are the most important for everyday medicine, but they’re far from the whole picture. The International Society of Blood Transfusion recognizes 47 blood group systems containing 366 different red cell antigens as of 2024. Most of these rarely cause problems in transfusions, but some do matter in specific situations.
One notable example is the Bombay phenotype, an extremely rare blood type found in roughly 1 in 10,000 people in parts of India and 1 in 1,000,000 in Europe. People with this phenotype lack a foundational antigen that the A and B antigens are built on, so their blood appears to be type O in routine testing but is actually incompatible with true type O blood. They can only receive blood from other people with the same Bombay phenotype, making donation networks for this group a serious logistical challenge.