Blood Types

There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called antigens (also called agglutinogens) on the surface of the RBC.

There are two antigens, A and B. If you have the A antigen on the RBC, then you have type A blood. When B antigen is present, you have type B blood. When both A and B antigens are present, you have type AB blood. When neither are present, you have type O blood.

When an antigen is present on the RBC, then the opposite antibody (also called agglutinin) is present in the plasma. For instance, type A blood has anti-type-B antibodies. Type B blood has anti-type-A antibodies. Type AB blood has no antibodies in the plasma, and type O blood has both anti-type-A and anti-type-B antibodies in the plasma. These antibodies are not present at birth but are formed spontaneously during infancy and last throughout life.

To make everything easier, this is how donations work (this without considering the Rh factor):

• A can donate to A, AB
• B can donate to B, AB
• 0 can donate to 0, A, B, AB
• AB can only donate to AB.