There are 8 common blood types most people know, but the full picture is much larger. The International Society of Blood Transfusion currently recognizes 48 blood group systems encompassing 398 different antigens on the surface of red blood cells. For everyday medical purposes, though, the number that matters is 8: the four ABO types (A, B, AB, and O) each combined with a positive or negative Rh factor.
The 8 Common Blood Types
The system you’re probably familiar with combines two classification methods. The first is ABO, which sorts blood by whether red blood cells carry the A antigen, the B antigen, both, or neither. The second is the Rh system, which checks for a protein called the RhD antigen. Combine these two and you get eight types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
In the U.S., O-positive is the most common at 38% of the population. AB-negative is the rarest, found in roughly 1% or fewer of people depending on ethnic background. Among Asian Americans, AB-negative occurs in just 0.1% of donors.
The distribution shifts significantly across populations. O-positive blood is found in 53% of Latin Americans but 37% of Caucasians. B-positive is common among Asian Americans (25%) but much less so among Latin Americans and Caucasians (9% each). These patterns reflect differences in how blood type genes have been passed down through generations in different populations.
How Blood Type Is Inherited
Your blood type comes from a single gene with three possible versions, called alleles: A, B, and O. You inherit one allele from each parent, giving you two copies. The A and B alleles each produce a specific molecule on the surface of your red blood cells. The O allele produces nothing functional at all.
A and B are codominant, meaning if you inherit one of each, both molecules show up on your cells and your blood type is AB. The O allele is recessive, so it only determines your blood type if you inherit two copies (one from each parent). Someone with type A blood could carry either two A alleles or one A and one O. This is why two parents with type A blood can sometimes have a child with type O: if both parents carry a hidden O allele, there’s a 25% chance their child inherits both.
Rh factor follows a similar pattern. Rh-positive is dominant, so you only need one copy of the RhD gene to test positive. Two Rh-positive parents can have an Rh-negative child if both carry one copy of the recessive version.
Why 48 Blood Group Systems Exist
Beyond ABO and Rh, scientists have identified 46 other blood group systems. Each one involves different proteins or sugar molecules sitting on the surface of red blood cells, and each is controlled by its own gene or set of genes. Most of these systems rarely cause problems in transfusions, which is why you’ve never heard of them.
The most recently recognized system, called MAL, was identified by researchers at the University of Bristol and NHS Blood and Transplant. They traced a mysterious blood antigen known as AnWj to a protein called Mal on red blood cell membranes. People who lack this protein (due to a deletion in the MAL gene) are AnWj-negative, a very rare phenotype. Discoveries like this continue to chip away at a small remaining group of blood antigens whose genetic basis hasn’t yet been explained.
For most transfusions and medical care, only ABO and Rh matter. But for patients who receive many transfusions over time, such as those with sickle cell disease, matching across additional systems becomes important to prevent the immune system from reacting to unfamiliar antigens.
Transfusion Compatibility
The rules for who can donate to whom depend on what’s being transfused. For red blood cells, the key is what antigens are on the donated cells. Type O-negative red blood cells carry no A, B, or RhD antigens, so they won’t trigger a reaction in any recipient. That’s why O-negative is used in emergencies when there’s no time to test. On the receiving end, AB-positive individuals can accept red blood cells from any type because their immune system already tolerates all those antigens.
Plasma works in reverse. Since plasma contains antibodies rather than red blood cells, the concern flips. AB plasma contains no anti-A or anti-B antibodies, making it safe for any recipient. O plasma, by contrast, contains both types of antibodies and can only go to other type O recipients.
How Blood Typing Works
Labs determine your blood type using two complementary tests. In forward typing, your red blood cells are mixed with lab-made antibodies against A, B, and RhD antigens. If the cells clump together when exposed to anti-A antibodies, you have the A antigen. The second test, reverse typing, flips the process: your plasma is mixed with known A and B red blood cells to confirm which antibodies you naturally carry. The two tests should agree. If forward typing says you’re type A, reverse typing should show anti-B antibodies in your plasma.
Rh Factor and Pregnancy
Rh status takes on special importance during pregnancy. If a mother is Rh-negative and her baby is Rh-positive (inheriting the RhD gene from the father), small amounts of fetal blood entering the mother’s circulation can trigger her immune system to produce antibodies against the RhD antigen. This usually causes no problems during the first pregnancy. The danger comes in subsequent pregnancies: those antibodies can cross the placenta and attack the red blood cells of another Rh-positive baby, causing a condition called hemolytic disease of the newborn.
The consequences range from mild anemia to severe complications including heart failure or fetal death. Prevention is straightforward and highly effective. Rh-negative mothers receive an injection of Rh immune globulin at 28 weeks of pregnancy and again within 72 hours after delivery if the newborn tests Rh-positive. This injection essentially clears any fetal Rh-positive cells from the mother’s system before her immune system has a chance to react, preventing the dangerous antibody buildup for future pregnancies.