A blood transfusion is a medical procedure when whole blood (containing both red blood cells and plasma) from a donor is given intravenously to a recipient. It is very common in long surgical procedures (particularly transplants), where a patient has suffered a severe blood loss, or where the patient is suffering from severe anemia.
Blood must be "typed" before transfusion into one of eight different blood types, categorized by the presence or absence of one of three different antibodies, A, B and Rhesus factor. The eight types are A+, A-, B+, B-, AB+, AB-, O+ and O-. O- is the most common blood type, AB+ the least common. However, the blood types do not have to match exactly - the only requirement is that the recipient not have a factor that is present in the donor blood. As such O- donors can donate to everyone, while AB+ can receive from everyone. However, it is not usually advisable to give a person too much of a non-type matched blood.
Giving blood is usually totally safe for anyone in good health, but must be avoided by those who harbor an infection, are suffering from allergies (which raise the white blood cell count) or are anemic. Receiving blood is not without risks. Although screening procedures are very good, there have been several cases where people have contracted diseases from donated blood. Author Isaac Asimov died from AIDS contracted from a transfusion, and former Toronto, Ontario mayor Mel Lastman was exposed to hepatitis C in the same way.
Although blood can be transfused directly, most transfused blood is stored from donors in refrigerated conditions and is used as required. Whole blood will last about thirty days before it can no longer be used for transfusions, requiring a constant stream of new donors.
Some religious groups will refuse blood transfusions on religious grounds, the best known of these groups being the Jehovah's Witnesses.
In You Don't Want to Know, a patient appeared to have been given the wrong blood type until House realized that instead he was producing antibodies that didn't match his blood type, a symptom of Lupus.