Blood Types Explained: ABO Groups, Rh Factor & All 8 Types

Quick answer

Your blood type is defined by two systems: ABO (whether your red cells carry the A antigen, the B antigen, both, or neither) and Rh (whether they carry the RhD antigen, shown as + or −). Combining them gives the eight common types. Blood type is inherited from both parents and can only be confirmed by a laboratory test.

The eight blood types

Select a type for a detailed page on its antigens, compatibility and inheritance.

The ABO system

The ABO system describes which sugar molecules (antigens) sit on the surface of your red blood cells. Your immune system makes antibodies against whichever ABO antigens you lack, which is why mismatched transfusions are dangerous.

ABO groups: antigens and antibodies
Group	Red cell antigens	Plasma antibodies	Genotypes
A	A	anti-B	AA or AO
B	B	anti-A	BB or BO
AB	A and B	none	AB
O	none	anti-A and anti-B	OO

The Rh factor

The Rh system includes many antigens, but “Rh-positive” or “Rh-negative” refers to the most important one, RhD. About 85 percent of people in the United States are Rh-positive. Rh-negative people do not naturally have anti-D antibodies but can develop them after exposure to Rh-positive blood, which is why Rh matters in transfusion and pregnancy.

How blood type is inherited

You receive one ABO allele and one Rh allele from each parent. A and B are codominant, O is recessive, and RhD-positive (D) is dominant over RhD-negative (d). This means two parents can often have children with blood types different from their own. Our baby blood type calculator works through every combination, and the ABO inheritance guide explains the genetics in more depth.

Frequently asked questions

What is the most common blood type?

O positive is the most common blood type in the United States, found in roughly 38 percent of people, followed by A positive at about 34 percent.

What is the rarest blood type?

AB negative is the rarest of the eight common types, at about 1 percent of the U.S. population. Frequencies vary considerably between ethnic groups and countries.

Can my blood type change?

Under normal circumstances, no. Apparent changes are rare and usually result from bone marrow or stem cell transplantation, certain cancers, or a laboratory error in an earlier test.

How do I find out my blood type?

A laboratory blood test, often done when donating blood or during prenatal care, is the only reliable way. Home test kits exist but should be confirmed by a laboratory before being relied upon.

Compatibility chartWho can donate to whom for red cells and plasma.Universal donorWhy O negative is the emergency red cell type.Universal recipientWhy AB positive can receive any red cell type.

Blood types explained