Different blood types are caused by variations in a single gene that controls which sugar molecules sit on the surface of your red blood cells. You inherit one copy of this gene from each parent, and the combination you receive determines whether your blood type is A, B, AB, or O. A second, separate gene determines whether you’re Rh positive or negative, giving us the eight common blood types most people recognize.
The ABO Gene and Its Three Versions
Your blood type starts with the ABO gene, which comes in three versions (called alleles): A, B, and O. Since you inherit one copy from each parent, your particular combination produces your blood type. The A and B versions are codominant, meaning if you inherit one of each, both are fully active, and your blood type is AB. The O version is recessive, so it only shows up as type O when you inherit it from both parents.
This means someone with type A blood could carry either two A copies or one A and one O. The same applies to type B. Two people who are both type A could have a child with type O if they each carry a hidden O allele. This is why blood type inheritance sometimes surprises families.
What Actually Makes Blood Types Different
The real action happens on the surface of your red blood cells. Every person starts with a base molecule called the H antigen sitting on those cells. The A and B gene versions each produce an enzyme that attaches a specific sugar to that H antigen, modifying it into either the A antigen or the B antigen.
The A enzyme adds a sugar called N-acetylgalactosamine. The B enzyme adds a different sugar, galactose. If you have both enzymes (type AB), both sugars get attached. The O version of the gene produces a nonfunctional enzyme that can’t add anything, so the H antigen stays unmodified. That’s all type O really is: undecorated red blood cells.
These sugar differences are tiny at a molecular level, but your immune system treats them as completely distinct identity tags.
Why Your Body Attacks the Wrong Blood Type
Your immune system produces antibodies against whichever ABO antigens your own red blood cells lack. This happens spontaneously during the first few months of life, likely triggered by exposure to similar sugars found on bacteria in your gut. You don’t need a bad transfusion to develop these antibodies. They form on their own.
The pattern follows a simple rule known as Landsteiner’s law:
- Type A carries A antigens on red cells and produces antibodies against B
- Type B carries B antigens and produces antibodies against A
- Type AB carries both antigens and produces neither antibody
- Type O carries neither antigen and produces antibodies against both A and B
This is why transfusion matching matters so much. If type A blood enters someone with type B, their pre-existing antibodies immediately attack those foreign red blood cells, triggering a potentially fatal reaction.
The Rh Factor: A Second Layer
The positive or negative label in your blood type comes from a completely separate gene system on chromosome 1. Two genes, RHD and RHCE, sit side by side and encode proteins on red blood cells. The most clinically important one is the D antigen, produced by the RHD gene. If your red blood cells carry the D antigen, you’re Rh positive. If they don’t, you’re Rh negative.
The most common reason someone is Rh negative is straightforward: the entire RHD gene is missing. In people of European descent, this deletion is the primary cause. In people of African descent, Rh negativity can also result from an inherited copy of the gene that contains errors preventing it from producing a working protein. About 77% of the population is Rh positive.
Unlike ABO antibodies, Rh antibodies don’t form spontaneously. An Rh-negative person only develops anti-D antibodies after being exposed to Rh-positive blood, either through a transfusion or during pregnancy. This is why Rh incompatibility between a pregnant person and their fetus can become a problem in second or later pregnancies, after the first exposure has already triggered antibody production.
How Blood Types Distribute Globally
Blood type frequencies vary by population, but data from NHS Blood Donation gives a useful snapshot. O positive is the most common at about 36%, followed by A positive at 28%. B positive accounts for roughly 8%, and AB positive is the rarest common type at just 2%. When you factor in Rh-negative versions, O negative makes up about 14%, A negative 8%, B negative 3%, and AB negative only about 1%.
These percentages shift significantly across different ethnic groups and geographic regions. Type B is more common in Central and South Asia. Type O reaches its highest frequencies in Central and South America. These patterns reflect thousands of years of evolutionary pressures acting on different populations.
Why Evolution Kept Multiple Blood Types Around
If one blood type were clearly superior, natural selection would have eliminated the others long ago. Instead, different blood types appear to offer advantages against different infectious diseases, keeping all of them circulating in the gene pool.
Malaria is considered the most significant selective force affecting blood group expression. Red blood cells that lack certain blood group molecules, or carry altered forms of them, are commonly found in regions where malaria is endemic. In Africa, specific variants in blood group systems beyond ABO are widespread precisely because they offer some protection against malaria parasites.
Other infections play a role too. Variations in ABO and related antigen systems influence susceptibility to Helicobacter pylori (the bacterium behind most stomach ulcers), norovirus, and cholera. Since different pathogens dominate in different parts of the world, no single blood type wins everywhere, and the diversity persists.
The Bombay Phenotype: When Type O Isn’t Really O
There’s a rare exception that reveals how the whole system works. Remember that every blood type starts with the H antigen as a base, and the A and B enzymes modify it. The H antigen itself is produced by a separate gene called FUT1. In extremely rare cases, both copies of FUT1 are broken, and no H antigen gets made at all.
Without the H antigen foundation, A and B antigens can’t be built either, even if the person carries functional A or B genes. This is called the Bombay phenotype, first discovered in Mumbai. It occurs in about 1 in 10,000 people in India and roughly 1 in a million in Europe. On standard blood tests, it looks like type O, but it’s fundamentally different. People with the Bombay phenotype produce antibodies against H, A, and B antigens, meaning they can only receive blood from other people with the same rare phenotype. Regular type O blood would trigger a reaction.
Far More Than Eight Blood Types
ABO and Rh get all the attention because they cause the most dangerous transfusion reactions, but they’re just two of 47 blood group systems recognized by the International Society of Blood Transfusion as of 2024, encompassing 366 distinct antigens. Most of these rarely cause clinical problems, but they can matter for people who receive frequent transfusions, such as those with sickle cell disease, where even minor mismatches can trigger immune responses over time.
Your blood type, in the fullest sense, is a complex mosaic of surface molecules shaped by the specific combination of genes you inherited. The familiar letter-and-sign label on your donor card captures only the two systems most likely to cause immediate harm if mismatched, but underneath that shorthand lies a much richer biological identity.