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In blood transfusions, medical personnel need to make sure a patient receives blood from a compatible donor. The relationship between blood group and blood transfusion is very important, as the wrong blood could cause severe illness. If possible, a patient in need of blood will be typed and cross-matched by care providers who will determine what kind of blood to give. If it is an emergency and the patient needs blood immediately, a doctor may authorize a transfusion with blood from an almost universally compatible donor.
There are a number of different blood group systems, broken up by different kinds of structures found on individual blood cells and in plasma. Two of the most important are the ABO and Rhesus groups, determined by surface molecules on the blood as well as plasma antibodies. A patient with type A blood has antibodies to B blood, and cannot receive transfusions from patients with B or AB blood. Conversely, patients with type B blood react to A and AB, while patients with AB blood can receive blood from type A or B patients.
Patients with type O blood have no surface molecules or antibodies, and their blood does not react with the A, B, or AB groups. They are considered “universal donors” because they can give blood to anyone. However, they can only receive blood from other people with type O, because they will react to type A, B, or AB blood.
The Rhesus blood group and blood transfusion also have an important relationship. Some patients have Rhesus D antigen in their blood, while others do not. This adds a layer of complexity to donor matching with the ABO system. If a patient has Rhesus D antigen, she can receive blood from a patient who shares it or doesn't have the antigen at all. Patients without the antigen, however, can get sick if they receive blood from a donor who carries it.
Thus, a patient could have A, B, AB, or O blood that may be positive or negative, depending on the presence of the Rhesus antigen. O blood is only a universal donor when the patient is Rhesus negative, as the blood can be safely infused into almost anyone. AB positive people are universal recipients, as they will not react to A, B, or O blood, and can receive a transfusion from a donor with or without the Rhesus antigen. This relationship between blood group and blood transfusion is important for care providers, as they need to select the most appropriate blood for their patients.
One problem with the focus on ABO and Rhesus blood group and blood transfusion issues is that there are a number of other compounds in the blood, classified under different grouping systems. These are much more rare and tend to primarily be a concern with specific human populations. If patient and donor have a conflict under one of these lesser-used systems, a seemingly compatible transfusion might actually cause a reaction.