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Blood type is a way of classifying the proteins on red blood cells in large groups of individuals that will invoke an immune system response if the blood is transfused to another individual.

The concept behind a blood transfusion to treat blood loss is ancient. However, for reasons that remained not understood, most transfusions resulted in widespread clotting and an inflammatory response that almost always resulted in the patient's death.

However, as experimentation continued, it was discovered that blood cells have protein covers that differ due to the genetic makeup of the person. Two of these, which were dubbed factor "A" and factor "B" were discovered in quick succession. Later, a third, called "Rh" after the Rhesus monkeys where it was first isolated, was also discovered. Any given individual may have none, one, any two, or all three of these proteins on the surface of their own red blood cells. A person is assigned type "O" has neither "A" nor "B" proteins, and the "Rh" factor is indicated by a "negative (-)" or "positive (+)". As such, a person with none of the proteins is "O-" (Oh-negative) and a person with all three is "AB+" (Ay-Bee-positive).

A person may receive a transfusion from anyone who does not have an incompatible protein on their red blood cells (although in practice, as much as possible, a person is only given blood of exactly the same type). People with type O- may therefore be donors to any other person, whereas persons with type AB+ may get a transfusion from any other donor. If a person gets a mismatched type, the immune system will treat the blood as a foreign material and provoke a full immune response.

Blood types are unique to humans and, as a result, humans may not receive transfusions from any other animal.

Other parts of blood, such as plasma, platelets and other blood products do not need to be typed.

James Wilson's blood type is O-. Gregory House has blood type AB+. Chris Taub's blood type is O-.

In the episode You Don't Want to Know, the patient was given type AB+ blood when he was really type A+. His lupus caused a false positive on the test for blood factor B.

Blood type at Wikipedia