Humans haven't always had different blood types, but rather evolved to have them over time in response to disease. The reason individual humans have different blood types is that they inherit the trait that their blood has certain antigens on the outside of their parents' blood cells. Antigens are molecules that trigger an immune response. There are actually more than four blood types: the International Society for Blood Transfusion (ISBT) recognizes 29 different blood group systems, encompassing more than 600 different antigens. However, the most well-known and medically useful blood grouping system, called ABO, distinguishes between four blood types.
Not all human blood types have always existed. In fact, they developed over time due to genetic changes and mutations. Type A is believed to be the oldest blood type, which later changed to type B about 3.5 million years ago. About a million years later, type O developed. Some people also have type AB blood, which has both A and B antigens. The reason the blood changed is thought to be in response to disease; for example, type O blood appears to be more resistant to malaria, and many people in malaria-prone areas have type O blood.
ABO blood types are important because blood of a type other than one's own can cause a fatal immune response if introduced into the body. Along with the antigens on the red blood cells, the body carries antibodies in the immune system that recognize and fight foreign antigens in the blood. The ABO system deals with the presence or absence of two specific antigens, called A and B.
Red blood cells can have A or B antigens, or both A and B antigens, or neither. Blood with A antigens alone is classified as type A, while blood with only B antigens is type B. Blood with both A and B antigens is classified as type AB, and blood without either antigen is type O.
The immune system has antibodies that protect against antigens that are not present in the body's own blood. Those with type A blood have anti-B antibodies; type B blood has anti-A antibodies; AB blood has neither of these antibodies, and people with type O blood have both. Patients cannot receive blood transfusions from donors whose blood contains an antigen that their own blood does not receive, because their antibodies will mount a defense against it. Ideally, both the donor and the recipient in a blood transfusion should have the same blood type. However, if this is not possible, other combinations are safe.
Type O blood is known as the "universal donor." Since it does not have A or B antigens, it can exist in the presence of anti-A and anti-B antibodies and is therefore compatible with any recipient blood type. In contrast, type O people can only receive blood from another type O person.
Type AB blood is also called "universal recipient." People with type AB blood do not have anti-A or anti-B antibodies, so they can accept any type of ABO blood from a donor. However, those with AB blood can only donate blood to recipients with the same blood type. Blood types A and B are mutually incompatible for blood transfusions, but both can donate to a type AB recipient or receive blood from a type O donor.
The ABO blood group system is often complemented by the Rhesus blood group system. Although the latter system deals with five specific antigens, it defines only two significant blood types, based on the presence or absence of antigen D: RhD negative and RhD positive. Those with blood types in the ABO system are sometimes said to be positive or negative, as in "type O negative," since RhD-negative recipients are not compatible with RhD-positive donors.