Humans evolved different blood types in response to disease. The ABO system distinguishes four blood types, but there are actually 29 different blood group systems with over 600 distinct antigens. Type O is the universal donor, while type AB is the universal recipient. The Rhesus blood group system is often integrated with the ABO system.
Humans didn’t always have different blood types, but rather evolved to have them over time in response to disease. The reason that individual humans have different blood types is that they inherit from their parents the trait of their blood that has certain antigens on the outside of the blood cells. Antigens are molecules that activate an immune response. There are actually far more than just four blood groups: The International Society of Blood Transfusion (ISBT) recognizes 29 different blood group systems, comprising over 600 distinct antigens. However, the best-known and most medically useful blood grouping system, called the ABO, distinguishes four blood types.
Not all human blood types have always existed. In fact, they have developed over time due to genetic changes and mutations. Type A is thought to be the oldest blood type, which then mutated into type B about 3.5 million years ago. About a million years later, type O developed. Some people also have blood type AB, which has both A and B antigens. The reason the blood changed is thought to be in response to disease: e.g. 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 different type than your 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 the antigens in the foreign blood. The ABO system deals with the presence or absence of two specific antigens, called A and B.
Red blood cells can have either A or B antigens, or both A and B antigens, or neither. Blood with only A antigens 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 with neither antigen is type O .
The immune system has antibodies that protect against antigens that are not present in the body’s blood. Those who have 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, because their antibodies will mount a defense against it. Ideally, both the donor and recipient in a blood transfusion should have the same blood type. If that’s not possible, however, other combinations are safe.
Type O blood is known as a “universal donor”. Since it carries neither A nor B antigens, it can exist in the presence of both anti-A and anti-B antibodies and is therefore compatible with any recipient blood group. In contrast, type O people can only receive blood from another type O person.
Blood type AB is also called the “universal recipient”. People with blood type AB have neither anti-A nor 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 integrated with the Rhesus blood group system. While the latter system deals with five specific antigens, it only defines two significant blood groups, based on the presence or absence of the D antigen: 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,” as RhD negative recipients are not compatible with RhD positive donors.
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