Blood Type Chart: ABO, Rh, and the Blood Typing Test

Your blood type is an identity card for your red blood cells. It isn't a number that runs high or low — it's a category, fixed by the antigens sitting on the surface of your red cells. Two systems do almost all the work in everyday medicine: the ABO system (types A, B, AB, and O) and the Rh system (positive or negative). Together they define the 8 blood types on every blood type chart, and they decide who can safely receive whose blood. This guide walks through what sets your type, the full blood type chart, how common each type is in the US, how the blood typing test is actually run, and why it all matters — with links to the deeper topics.

Key takeaways

• Your blood type is set by antigens on your red blood cells; the two systems that matter most are ABO (A, B, AB, O) and Rh (+ / −).¹²
• ABO gives four groups by whether you carry the A and/or B antigen — and your plasma carries natural antibodies against the antigens you lack.¹
• Rh makes you positive or negative depending on one antigen, the D antigen; about 85% of Americans are Rh-positive.³⁴
• Combining ABO and Rh gives the 8 blood types: O+, O−, A+, A−, B+, B−, AB+, AB−. In the US, O+ and A+ are most common; AB− is the rarest of the eight.⁵⁴
• The blood typing test is a simple blood draw checked two ways — forward typing (your cells vs. known antibodies) and reverse typing (your plasma vs. known cells).⁶
• Your type is inherited and does not change through life; it governs transfusion safety and pregnancy monitoring — see the deep dives on compatibility, the Rh factor, and pregnancy.

What determines your blood type?

On the surface of every red blood cell sit molecules called antigens. Which antigens you carry — inherited from your parents — is what places your blood in a group. Antigens matter because your immune system builds antibodies against the ones you don't have. Those antibodies are exactly what make one person's blood incompatible with another's during a transfusion.¹

There are actually more than 40 recognized blood group systems and hundreds of red-cell antigens catalogued, but two systems dominate clinical practice.⁷

The ABO system

Described at the start of the 20th century, ABO is the single most important system for transfusion.¹ It's built on two antigens, A and B:

• Type A — the A antigen sits on your red cells, and your plasma carries anti-B antibodies.
• Type B — the B antigen, with anti-A antibodies.
• Type AB — both A and B antigens, and no anti-A or anti-B antibodies.
• Type O — neither antigen, but both anti-A and anti-B antibodies.

The defining quirk of ABO is that these antibodies are naturally occurring: they're present without any prior transfusion, which is why an ABO mismatch can trigger an immediate, dangerous reaction.¹ Under the hood, the A and B antigens are sugar chains built onto a common precursor called the H antigen — type O still carries H, just without the A or B sugar added on top. Whether you make the A or B enzyme is set by the ABO gene on chromosome 9, and a handful of weaker subgroups (such as A2) exist, which is one reason the typing test is always cross-checked.⁷

The Rh system

The second key system is Rh, and in everyday use it comes down to a single antigen, the D antigen:³

• carry the D antigen and you are Rh-positive (Rh+);
• lack it and you are Rh-negative (Rh−).

Unlike ABO, there are no natural anti-D antibodies. They only appear after exposure to Rh-positive blood — through an incompatible transfusion or a pregnancy — which is the whole reason the Rh factor matters so much in pregnancy. Your full type is written by combining the two systems: O+, A−, AB+, and so on.

The 8 blood types (chart)

Putting ABO and Rh together yields the eight types on any blood type chart. The table below shows the antigens each type carries, the antibodies in its plasma, roughly how common it is among Americans, and — in broad strokes — who it can give red cells to and receive them from.¹²⁵

Two patterns fall out of the chart. O− carries no ABO or Rh antigens, so its red cells provoke no reaction — the universal red-cell donor. AB+ carries every antigen and makes no anti-A/anti-B antibodies, so it can receive from anyone — the universal recipient. (Plasma runs the opposite way: AB is the universal plasma donor.) The full rules — including why plasma and platelets follow their own logic — are laid out in blood type compatibility.

How common is each type (US)

Blood type frequencies vary by ancestry, so national figures blend a diverse population.⁴ Using American Red Cross estimates, the rough US breakdown is:⁵

• O-positive — ~38% (the most common single type)
• A-positive — ~34%
• B-positive — ~9%
• O-negative — ~7%
• A-negative — ~6%
• AB-positive — ~3%
• B-negative — ~2%
• AB-negative — ~1% (the rarest of the eight)

Grouped by system, roughly 45% of Americans are type O, 40% type A, 11% type B, and 4% type AB; about 85% are Rh-positive and 15% Rh-negative.⁴³ These proportions differ sharply across ethnic groups — for example, type B and Rh-negative status are far more or less common depending on ancestry — which is why blood centers actively recruit diverse donors to match patients.⁴

A note on "rare." AB− is the least common of the eight everyday types, but true rare blood means lacking a high-prevalence antigen most people carry — phenotypes tracked by rare-donor registries, such as the Bombay type, which lacks the A, B, and H antigens entirely and can only receive Bombay blood.⁷

The blood typing test

The blood typing test is done in a lab on an ordinary blood draw — no fasting, no special prep. To make the result reliable, technicians confirm it two ways that must agree.⁶

• Forward typing — your red cells are mixed with reagent antibodies (anti-A and anti-B). If your cells clump (agglutinate) with anti-A, you carry the A antigen; clump with anti-B, you carry B.
• Reverse (back) typing — your plasma is mixed with known type A and type B cells. Because your plasma holds the antibodies against antigens you lack, the reactions should mirror the forward result. A type O person, for instance, agglutinates both.
• Rh typing — your cells are tested against anti-D reagent; clumping means Rh-positive.

The clumping technicians look for is agglutination — antibodies bridging red cells into visible clusters. It's a yes/no reaction, which is why blood type is categorical rather than a number on a scale. Because forward and reverse typing cross-check each other, a disagreement flags a problem — an antibody issue, a subgroup, or a labeling error — before any blood is released.

When a transfusion is likely, the lab runs what's called a type and screen: your ABO/Rh type plus an antibody screen that hunts for antibodies against the many red-cell antigens beyond ABO (such as those in the Kell, Duffy, and Kidd systems), which can develop after past transfusions or pregnancies. If antibodies turn up, finding compatible units gets harder — and this is where DNA-based genotyping increasingly helps, mapping antigens precisely for patients who need repeated transfusions or complex matches.⁸ Just before release, the lab performs a crossmatch, physically testing donor cells against your plasma as a final safety check.⁶ You can learn your own type through your doctor or by giving blood — see blood donation.

Why your blood type matters

Knowing your type is not trivia — it drives real clinical decisions.

• Transfusion. Giving ABO-incompatible red cells can cause a severe, sometimes fatal reaction, because natural anti-A/anti-B antibodies attack the donor cells at once. Matching type is fundamental to a safe blood transfusion, and mismatch is a leading preventable transfusion hazard.⁹
• Pregnancy. An Rh-negative mother carrying an Rh-positive baby can form anti-D antibodies that endanger this or a future pregnancy — hemolytic disease of the fetus and newborn. It's largely preventable with Rh immune globulin, which is why every pregnancy includes blood typing and antibody screening.¹⁰ The details are in blood type and pregnancy.
• Donation and supply. Compatibility shapes how donated blood is used — O− for emergencies, AB plasma as a universal option — so blood centers track the mix constantly (universal blood donor).²
• Inheritance. Your type follows predictable genetic rules, which is why parents' types constrain a child's — the logic behind blood type inheritance.⁷

Can your blood type change?

For practical purposes, no. Your blood type is written in your DNA and stays the same for life. A result that "changes" between tests almost always means a labeling or clerical error, not a real shift — one more reason typing is confirmed on two samples.⁶

There are rare, genuine exceptions in medicine: a bone-marrow or stem-cell transplant can switch a recipient's type to the donor's over time, and some cancers or infections can transiently weaken the expression of an antigen. And blood-group science is still moving — in 2024 researchers defined an entirely new system, MAL (the AnWj antigen), resolving a 50-year puzzle and proving the map of human blood groups isn't finished.¹¹ But none of this changes the everyday truth: the type on your card is yours for good.

Frequently asked questions

Bottom line

Your blood type is the identity card of your red blood cells, set mainly by the ABO system (A, B, AB, O) and the Rh factor (+ / −). Together they make the 8 types on every blood type chart — with O+ and A+ the most common in the US and AB− the rarest. The blood typing test confirms your type two ways, forward and reverse, on a simple blood draw. Your type is inherited, doesn't change, and matters most for transfusion and pregnancy. To go further, explore compatibility, the Rh factor, inheritance, and pregnancy — and have your results read in full context with AI DiagMe, alongside your physician.

Sources

Official US sources and peer-reviewed publications (PubMed) used for this guide: