The ABO/Rh Blood System
In the early 1900s, a scientist named Karl Landsteiner discovered that there are four main blood types in humans. These blood types are determined by special markers called antigens on the surface of red blood cells. The two main antigens are called A and B.
After birth, the body starts making antibodies—proteins that attack foreign substances—against the antigens that are not present on a person’s own red blood cells. For example:
A person with Type A blood has A antigens and makes anti-B antibodies.
A person with Type B blood has B antigens and makes anti-A antibodies.
A person with Type AB blood has both A and B antigens and does not make anti-A or anti-B antibodies.
A person with Type O blood has no A or B antigens and makes both anti-A and anti-B antibodies.
The blood type is determined by the antigens, not the antibodies a person has.
In the U.S., the most common blood type is O (45%), followed by A (39%), B (12%), and the rarest type is AB (4%).
Later, in 1940, Landsteiner and another scientist, Wiener, discovered another set of antigens called Rh factors. These antigens were first found in Rhesus monkeys, which is why they are called Rh factors. A person who has these antigens is Rh+ (positive), while someone who does not have them is Rh- (negative).
Unlike the A and B antibodies, the body does not naturally have Rh antibodies. However, they can be made if an Rh- person is exposed to Rh+ blood, such as during a blood transfusion or when an Rh- mother is pregnant with an Rh+ baby.
