Blood Type Compatibility Chart: Who Can Donate to Whom (and How You Inherited Yours)
Three very different real-life questions all come down to the same two molecules sitting on the surface of your red blood cells: can this donated blood safely go into that patient, what blood type could two parents' children end up with, and why does a doctor watch an Rh-negative pregnancy so closely? Most charts online answer the first question and stop there, so an unfamiliar pairing — can AB- give plasma to O+? — turns into a guess instead of something you can work out. Here's the underlying mechanism, worked through with real numbers, so you can derive any answer instead of memorizing a grid.
The two systems stacked into "your blood type"
"Blood type" is actually two independent classifications reported together. The ABO system describes which of two antigens — A, B, both, or neither — sit on your red blood cells. The Rh system describes one more antigen, called D, which you either have (Rh-positive) or don't (Rh-negative). Combine the four ABO groups with the two Rh states and you get the eight familiar types.
| Blood type | Antigens on red cells | Antibodies naturally in plasma |
|---|---|---|
| O- | None | Anti-A, anti-B |
| O+ | D only | Anti-A, anti-B |
| A- | A | Anti-B |
| A+ | A, D | Anti-B |
| B- | B | Anti-A |
| B+ | B, D | Anti-A |
| AB- | A, B | None |
| AB+ | A, B, D | None |
Why a mismatch is dangerous
A transfusion goes wrong when a recipient's plasma antibodies meet the matching antigen on donated red cells: the antibodies latch on and clump the donor cells together, which can trigger a fatal reaction. Note the asymmetry in the table above — anti-A and anti-B antibodies are present in your plasma from early childhood whether or not you've ever had a transfusion, because they mirror antigens on the food-borne and bacterial particles everyone is exposed to early in life. Anti-D antibodies work differently: an Rh-negative person is not born with them. They only appear after that person's immune system has actually been exposed to Rh-positive blood, through a mismatched transfusion or a pregnancy. That distinction is the reason Rh incompatibility is manageable during a first pregnancy but becomes the real danger in a second one, covered below.
The red-cell chart, worked through
Take a patient who is B-negative and needs a transfusion. Their plasma already carries anti-A antibodies, so any donor cells carrying the A antigen (types A or AB) are ruled out immediately. Because the patient is Rh-negative, giving Rh-positive cells risks sensitizing them for a future transfusion or pregnancy, so Rh-positive donors are avoided too. That leaves exactly two safe donor types: B- and O-. Run the same logic on AB+: their plasma has neither anti-A, anti-B, nor (once already Rh-positive) any reason to reject D-positive cells, so all eight types are safe — which is why AB+ is called the universal recipient. Run it on O-: its cells carry no A, no B, and no D antigen, so no recipient's antibodies have anything to attack, making it the universal donor for red cells.
| Recipient | Can receive red cells from |
|---|---|
| O- | O- only |
| O+ | O-, O+ |
| A- | O-, A- |
| A+ | O-, O+, A-, A+ |
| B- | O-, B- |
| B+ | O-, O+, B-, B+ |
| AB- | O-, A-, B-, AB- |
| AB+ | everyone (universal recipient) |
Plasma runs the opposite direction
Plasma transfusion risk flows the other way: it's the donor's antibodies that can attack the recipient's red cell antigens. AB plasma has no anti-A or anti-B antibodies at all, so it can't attack anyone's cells regardless of their type — AB is the universal plasma donor. O plasma is the opposite case: it carries both anti-A and anti-B antibodies, so it's only safe for an O recipient, whose own cells have no antigens for those antibodies to find. And because O red cells carry no antigens, an O recipient can safely accept plasma from any type — O is the universal plasma recipient. Mix up the two charts and you'll give a patient exactly the wrong unit.
How you inherited your blood type
ABO type is set by one gene with three versions (alleles): I^A and I^B, which are codominant with each other, and i, which is recessive to both. Genotype I^AI^A or I^Ai gives type A; I^BI^B or I^Bi gives type B; I^AI^B gives AB (both antigens show up at once); ii gives O. Rh works on a separate gene entirely: D is dominant, d is recessive, so DD or Dd is Rh-positive and only dd is Rh-negative.
Here's the part that surprises most people: two parents who are both type A and both type B can still have a child of any of the four ABO types. Suppose the mother is type A but carries a hidden O allele (genotype I^Ai), and the father is type B, also carrying a hidden O allele (I^Bi) — neither parent shows it, since i is recessive. Each parent passes one of their two alleles at random, giving four equally likely combinations: I^AI^B (type AB, 25%), I^Ai (type A, 25%), I^Bi (type B, 25%), and ii (type O, 25%). An A parent and a B parent producing an O or AB child isn't a paperwork error — it's exactly what the genetics predicts once you know both parents are carriers.
Add Rh to the same example. If both parents are Rh-positive but heterozygous (Dd), each passes D or d with equal odds, giving DD (25%), Dd (50%), or dd (25%) — so the child is Rh-positive 75% of the time and Rh-negative 25% of the time, even though neither parent is Rh-negative. Because the ABO and Rh genes sit on different chromosomes, they're inherited independently, so you multiply the two probabilities to get the full picture: for example, the chance of an A-negative child from these two particular parents is 25% (type A) x 25% (Rh-negative) = 6.25%.
Why this matters before a baby is even born
Rh incompatibility becomes a real medical concern in a specific setup: an Rh-negative mother carrying an Rh-positive fetus (possible whenever the father is Rh-positive). During delivery — or a miscarriage, or an invasive prenatal procedure — small amounts of the baby's Rh-positive blood can cross into the mother's circulation. Her immune system, seeing the D antigen for the first time, starts producing anti-D antibodies. This first pregnancy is usually unaffected, because sensitization happens mostly at delivery, after the baby is already born. The danger is a later pregnancy: if that next fetus is also Rh-positive, the mother's now-primed anti-D antibodies can cross the placenta and destroy the fetus's red blood cells, a condition called erythroblastosis fetalis (hemolytic disease of the newborn). The routine prevention is an injection of anti-D immune globulin given to Rh-negative mothers during pregnancy and again after delivery, which clears any stray fetal Rh-positive cells from the mother's blood before her immune system can mount its own response.
This page explains the underlying biology and genetics for educational purposes; it isn't medical advice, and any actual blood typing, transfusion decision, or prenatal Rh management should go through a laboratory and a clinician, not a chart.
Test what you just learned against real scenarios in these human body trivia questions.
Source: Encyclopaedia Britannica, "ABO blood group system," "Rh blood group system," "Blood typing," "Blood group," and "Erythroblastosis fetalis" entries.