Why are there different types of blood?

During a crisis on a television hospital show, patients stream into the emergency room. Not knowing the patient’s blood type, the doctor calls for O-negative. In most ways, everyone’s blood is the same. It consists of two main types of cells—red and white—suspended in a clear salty liquid—plasma. On a microscopic level, however, the red blood cells are not all alike. If the wrong types are mixed together, they form clumps of cells that can interfere with blood flow, with possibly fatal results. Why do people have different types of blood?

The difference in blood types is a result of chains of different sugar molecules that are attached to the surface of red blood cells. These attachments, known as antigens, identify the blood cell to the immune system. Two distinct types of antigens exist in human blood, labeled A and B. If a red blood cell has the A antigen, it is labeled Type A. There are four possible blood types: Type A, Type B, Type AB (both antigens), and Type O (neither antigen). If blood containing one of the antigens is transfused into a person whose cells do not have that antigen, the immune system reacts as if the new cells were an invading infection. As the body attempts to destroy the invaders, the blood forms dangerous clumps. After the development of the ABO system of classification, a third antigen, called the Rh factor (because it was first observed in the blood of rhesus monkeys) was discovered.

Red cells with the antigen are called Rh positive and those without it, Rh negative. Combining this classification with the ABO system gives eight distinct blood types. Because Type O negative blood has none of the antigens, it can be given to a person of any blood type without risk of an immune response, so people with O-negative blood are called universal donors. If you have Type AB-positive blood, your immune system does not respond to any of the antigens, and you are a universal receiver.

Blood type is a genetic trait, inherited from your parents. A parent with Type A blood can have children with Type A or Type B, but not Types AB or O.

Although it is not certain why the blood types developed, the frequency of particular types of blood varies among populations originating in different places on Earth. Research indicates that blood type may be linked to susceptibility or vulnerability to certain diseases. For example, people with B or O blood have a slightly lower risk of certain cancers, while people with the A or B antigens (or both) have a lower risk of contracting cholera or plague. This has led to researchers believing antigens may have developed as part of the body’s immune response to diseases.

In 2007, an international research team announced that they had discovered an enzyme that can remove the antigens from red blood cells. Although extensive testing will be necessary before this process becomes acceptable for general use, it may allow any type of blood to become Type O. This would greatly increase the efficiency of blood banks and the safety of transfusions.