Blood Types Chart: A, B, AB & O Explained with Full US Distribution Data

What Are Blood Types — and Why Do They Matter?

Blood types might seem like a simple label, but they determine who can safely receive blood from whom — and getting it wrong can be fatal. The system was cracked open in 1901 by Austrian physician Karl Landsteiner, who identified three blood groups (A, B, and O) based on molecules called antigens on the surface of red blood cells. A fourth type, AB, was identified just one year later in 1902.

Before this discovery, early blood transfusion attempts in the 1600s often ended in death — partly because doctors tried using animal blood in human patients. Landsteiner’s work transformed transfusion medicine and earned him the Nobel Prize in 1930. Today, blood typing is one of the first tests run in any medical emergency.

How the ABO System Works

The ABO system classifies blood based on which antigens are present on your red blood cells and which antibodies float in your plasma. There are four main groups:

• Type A — has A antigens on red cells; has anti-B antibodies in plasma
• Type B — has B antigens on red cells; has anti-A antibodies in plasma
• Type AB — has both A and B antigens; has neither antibody (can receive any type)
• Type O — has neither antigen; has both anti-A and anti-B antibodies (can donate to any type)

When incompatible blood is transfused, the recipient’s antibodies attack the donor’s red blood cells — triggering a reaction that can cause shock, kidney failure, or death. This is why matching matters.

What Is the Rh Factor?

Every ABO blood type comes with an additional marker: the Rh factor. If your red blood cells carry the Rh antigen (also called the D antigen), you’re Rh positive (+). If not, you’re Rh negative (−). This gives us 8 blood types total: A+, A−, B+, B−, AB+, AB−, O+, and O−.

Rh factor becomes especially important during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, her immune system can develop antibodies against the baby’s blood — a condition called hemolytic disease of the newborn. This is why Rh compatibility is routinely checked at prenatal appointments.

Blood Type Distribution in the United States

How common is your blood type? Here’s how the 8 types break down across the American population:

O+ and A+ together account for over 70% of the population. If you have one of those types, you’ll almost always find compatible blood when you need it. If you’re AB−, you’re in rare company — but you can receive from any Rh-negative donor.

Who Can Donate to Whom? (Compatibility Summary)

The chart above only tells part of the story. Here’s a quick summary of the key compatibility rules:

• O− (universal donor) — red blood cells can be given to anyone in emergencies, though cross-matching is always preferred
• AB+ (universal recipient) — can receive red blood cells from any type
• AB (plasma) — AB plasma can be given to patients of any type
• O (whole blood) — O donors can only give whole blood to other O recipients

For the full compatibility breakdown including plasma and whole blood rules, see our blood type compatibility chart.

Important update: The Mayo Clinic and other institutions note that O− blood is no longer considered automatically “safe for everyone” due to complex immune reactions beyond the ABO system. Modern transfusion medicine always cross-matches donor and recipient blood before transfusion — except in extreme, life-threatening emergencies where no time exists to test.

The Genetics Behind Your Blood Type

Your blood type is determined before you’re even born. You inherit one allele from each parent, and those two alleles together determine your ABO type. The three alleles in the system are A, B, and O — giving six possible genetic combinations:

Notice that A and B are both dominant over O, but codominant with each other. That means if you carry both an A and a B allele, both express — giving you type AB. O only shows up when both alleles are O.

This also means two type-A parents can have a type-O child (if both parents carry an AO combination and the child inherits O from each). For a deeper look at how inheritance works across parent-child combinations, see our article on blood type inheritance.

Why Blood Type Matters for Transfusion

Blood banks don’t just match ABO type. Over 600 additional antigens exist on human red blood cells, and any of them can trigger an immune response. For most routine transfusions, ABO and Rh matching is sufficient. But for patients who receive frequent transfusions — such as those with sickle cell disease or thalassemia — extended antigen matching is critical to prevent antibody buildup.

One practical note: spouses should generally not donate blood directly to each other during childbearing years. If a husband donates to his wife and she develops antibodies to his red cell antigens, a future pregnancy could be complicated if the fetus inherits those same antigens. Those maternal antibodies can cross the placenta and attack fetal red blood cells, causing anemia and jaundice.

Blood Type and Donation: Why Your Type Matters

Blood centers always need all types, but certain types are especially valuable:

• O−: Every hospital stockpiles O− for emergencies — making it perpetually in short supply
• AB−: Rare enough that those who have it are strongly encouraged to donate regularly
• AB+ plasma: The universal plasma donor — AB plasma can be given to anyone regardless of blood type

Only about 5% of healthy Americans donate blood, yet over 80% of blood used for transfusions comes from repeat donors. If you know your type and are eligible, the need is real.

Not sure what your blood type is? The next time you donate blood, you’ll receive your results — or ask your doctor to include it in routine bloodwork.