Different people have different types of blood running through their veins. Humans have four main blood groups: A, B, AB and O. These are defined by which antigens are present on the surface of red blood cells: Type A blood has the A antigen on red blood cells, B has the B antigen, AB has both and O has neither.
In general, the rarest blood type is AB negative and the most common is O positive. Here's a breakdown of the most rare and common blood types by ethnicity, according to the American Red Cross.
A person's blood type is based on whether or not they have certain molecules or proteins — called antigens — on the surface of their red blood cells, according to the National Institutes of Health. Two of the main antigens used for blood typing are known as "A antigen" and "B antigen." People with type A blood only have A antigens on their red blood cells and those with type B blood have only B antigens. Individuals with type AB blood have both; people with type O blood have neither.
Another protein, the "Rh factor" — also known as the "Rhesus" system — is also present or absent on red blood cells. A person's blood type is designated as "positive" if they have the Rh protein on their red blood cells, and "negative" if they don't have this protein.