How Rare Are the Different Blood Types?

Of the eight different common blood types, AB negative is the least common blood type in the United States, according to the American Red Cross. O positive is the most common type, followed by A positive as the second most common, and then B positive.

The fourth most common blood type among the U.S. population is AB positive, notes the American Red Cross. O negative, A negative and B negative are the fifth, sixth and seventh most common blood type, respectively.

Though the overall relative distribution of blood types is consistent across ethnic groups, each ethnic group has its own mix of the blood types, according to the American Red Cross. For example, for all ethnic groups, those with either O positive or A positive blood comprise more than 65 percent of all people. However, among African-Americans, more than 80 percent possess either O positive or A positive blood. For another example, 7 percent of Caucasians in the United States have A negative blood type compared to 0.5 percent of the Asian-American population.

Blood type is based on the presence or absence of three antigens, explains the American Red Cross. Four major blood groups are based on the A and B antigens, resulting in A, B, AB or O types. Each of these types is identified as positive or negative, depending on whether the third antigen, the RH factor, is present in the blood.