Your blood type is determined by molecules called antigens on the surface of your red blood cells, combined with a protein called the Rh factor. Together, these two systems produce the eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−. Which combination you carry is decided entirely by the genes you inherit from your parents.
The ABO System
The first half of your blood type comes from the ABO system, which sorts everyone into four groups based on which antigens sit on their red blood cells. Type A blood carries A antigens. Type B carries B antigens. Type AB carries both. Type O carries neither.
Your body also produces antibodies in your plasma that target whichever antigens you don’t have. If you’re Type A, your plasma contains antibodies against B antigens. If you’re Type B, your plasma attacks A antigens. Type AB produces no antibodies against either, which is why AB is the universal plasma donor. Type O produces antibodies against both A and B, but since O red blood cells carry no antigens at all, Type O negative is the universal red cell donor, safe to give in emergencies when there’s no time to check a patient’s type.
How You Inherit Your Blood Type
The gene responsible for your ABO blood type sits on chromosome 9 and comes in three versions (alleles): A, B, and O. You inherit one allele from each parent, giving you two copies total. The combination of those two alleles determines your blood type.
A and B are co-dominant, meaning if you inherit one of each, both are expressed and you end up with Type AB. The O allele is recessive, so it only shows up when you inherit two copies. This creates six possible genetic combinations:
- AA or AO produces Type A
- BB or BO produces Type B
- AB produces Type AB
- OO produces Type O
This is why two parents who are both Type A can have a child who is Type O. If each parent carries one A allele and one hidden O allele (AO genotype), there’s a one-in-four chance their child inherits O from both sides. It also means Type AB parents cannot have a Type O child, since neither parent has two O alleles to pass along.
The Rh Factor
The second half of your blood type, the positive or negative, comes from the Rh factor. This is a protein on the surface of red blood cells. If you have it, you’re Rh-positive. If you don’t, you’re Rh-negative. The Rh-positive trait is dominant, so you only need one copy of the gene to test positive. About 77% of people are Rh-positive.
Rh status matters most during pregnancy. If an Rh-negative mother carries an Rh-positive baby, fetal blood cells can cross into her bloodstream, especially during delivery. Her immune system may recognize the Rh protein as foreign and build antibodies against it. This usually doesn’t harm the first baby, but those antibodies persist. In a later pregnancy with another Rh-positive baby, the antibodies can cross the placenta and attack the baby’s red blood cells, causing a serious condition called hemolytic disease. To prevent this, Rh-negative mothers receive injections of Rh immune globulin during pregnancy and after delivery if the baby turns out to be Rh-positive. The medication stops the mother’s body from producing the harmful antibodies in the first place.
How Blood Type Is Tested in a Lab
Blood typing in a lab relies on a simple principle: when antibodies meet their matching antigen, red blood cells clump together. This visible clumping, called agglutination, is what technicians look for.
The standard test has two parts. In forward typing, a sample of your red blood cells is mixed with known antibodies against A, B, and Rh. If the cells clump when exposed to anti-A antibodies, you have A antigens. If they clump with anti-B, you have B antigens. If they clump with anti-D (the Rh antibody), you’re Rh-positive. No clumping at all means Type O negative.
Reverse typing works the other direction. Your plasma is mixed with known Type A and Type B red blood cells to confirm which antibodies you carry. The two tests should agree. If forward typing says you’re Type A, reverse typing should show your plasma clumps Type B cells but leaves Type A cells alone.
Labs run these tests using one of three methods. Tube testing is the traditional manual approach, with each reaction happening in a separate test tube. Gel column testing is more automated: red blood cells and antibodies are combined in tiny columns filled with gel, then spun in a centrifuge. Clumped cells get trapped at the top of the gel while unclumped cells sink to the bottom, making results easy to read. Solid phase testing uses microtiter plates coated with antigens or antibodies and adds indicator cells to detect whether binding occurred.
Blood Type Distribution
Blood types are not evenly distributed. Based on donor data from NHS Blood Donation, the breakdown looks like this: O positive is the most common at 36%, followed by A positive at 28%. B positive and O negative each account for about 8 to 14%. The rarest common type is AB negative, found in roughly 1% of donors.
These percentages shift significantly across ethnic groups and geographic regions. Populations in East Asia tend to have higher rates of Type B, while Indigenous populations in the Americas have very high rates of Type O. Your individual blood type reflects the genetic history of your ancestors as much as it reflects your own biology.
The Bombay Phenotype: A Rare Exception
There’s a rare twist in the ABO system that catches even experienced lab technicians off guard. The A and B antigens are built on top of a foundation molecule called the H antigen. The A gene converts H antigens into A antigens, the B gene converts them into B antigens, and Type O blood simply leaves most H antigens unconverted.
People with the Bombay phenotype carry a recessive genetic variant (hh) that prevents them from producing the H antigen at all. With no H antigen as a building block, they can’t make A or B antigens either, so they look like Type O on a standard blood test. But there’s a critical difference: their immune system recognizes the H antigen as foreign and produces antibodies against it. Since virtually all donated blood contains H antigens (even Type O), Bombay phenotype individuals are incompatible with almost every donor. They can only receive blood from other Bombay phenotype individuals, which is typically sourced from rare frozen inventories. The phenotype is extremely uncommon globally, though it occurs at somewhat higher rates in parts of South Asia.