Blood groups and blood types are classifications based on the levels of antibodies and inherited antigens in red blood cells. The International Society of Blood Transfusion acknowledges 36 human blood group systems and several hundred antigens. The two most important and common blood group systems are the ABO and Rh systems. The former determines blood type while the latter determines if the blood possesses the D antigen. This information is integral to blood transfusions.

ABO Blood Group System

The ABO blood group system is the one about which most people are aware. This system classifies blood based on the presence of the A and B antigens and antibodies. In this group, there are four types of blood. Type A contains the A antigen and the B antibody. Type B contains the B antigen and the A antibody. For type AB, the blood needs both A and B antigens, but neither antibody. Type O is the opposite -- neither antigen but both antibodies. The ABO blood group system is the single most important classification system in human blood transfusions. A mismatch in ABO typing can cause serious reactions and even death in the recipient.

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ABO Genetics

Like eye or hair color, a child inherits their blood type depending on their parents’. As long as the parents do not have type AB blood, the child has the potential to have type O blood. This is in addition to whichever blood type the parents have. For example, if the parents have type A and type B blood, the child could have type O, type A, or type B. However, if one or both of the parents have type AB blood, the child can have any blood type except type O.

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Rh Blood Group

The Rh blood group system is also vital for transfusion. This blood system categorizes blood based on the presence of the Rh(D) antigen. This antigen is significant because it can provoke an immune response if a blood transfusion is incompatible. If blood has the antigen, it is positive. If it lacks the antigen, it is negative. In conversations, individuals often refer to blood types as a fusion of ABO and Rh systems. Phrases such as “A negative” or “B positive” are common. In writing, the Rh type is typically noted by a + or - symbol.

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Rare Blood Types

Beyond the eight most common, there are several rare blood types. Because there are so many antigens in blood, the presence or absence of some of these antigens creates a rare blood type. For example, a person with no Rh antigens at all has type Rh null. A blood type counts as rare if fewer than one in 1,000 people possesses it. Some blood types are unique to racial or ethnic groups. Those with rare blood types will often bank their own blood because there is a lack of supply for transfusions.

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Determining Blood Type

The tests to determine blood type are simple. At a clinical laboratory or hospital, a technician will draw a few samples of blood from the person they are testing. They then mix one sample with type A antibodies and the other with type B antibodies. This allows the technician to determine the blood typing. For example, if the blood clumps together in the sample with the A antibodies, the patient has type B blood. The technician then performs a similar test for the Rh typing.

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There are many reasons for blood transfusions. Blood loss from injuries or surgeries can result in significant blood loss, after which it can take the human body weeks to replace the lost red blood cells fully. A blood transfusion can replace the lost blood more quickly, keeping blood quantity at a healthy level. However, some blood types can’t receive transfusions of other blood types -- this will trigger an immune system response that could increase the danger. This is why physicians always check blood types and ensure compatibility before beginning a transfusion.

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Bblood compatibility is a complex subject because of all the possible combinations.

  • Type A+ can donate to A+ and AB+. They can receive blood from A+, A-, O+, and O-.
  • Type A- can donate to A+, A-, AB+, and AB-. They can receive from A- and O-.
  • Type B+ can donate to B+ and AB+. They can receive blood from B+, B-, O+, and O-.
  • Type B- can donate to B+, B-, AB+, and AB-. They can only receive from type B- and O-.
  • Type AB+ can only donate to AB+. They are universal receivers.
  • Type AB- can donate to AB+ and AB-. They can receive from AB-, A-, B-, and O-.
  • Type O+ can donate to O+, A+, B+, and AB+, but can only receive blood from O+ and O-.
  • Type O- are universal donors, but can only receive blood from O-.

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The most common blood group is type O. The Red Cross states that 45 percent of the United States is type O+, though the exact percentage differs between ethnic and racial groups. Around 45 percent of Caucasian people are type O, compared to 51 percent of Black people and 57 percent of Hispanic people. Despite this, only seven percent of people in the United States are type O- -- O+ making up the vast majority. The rarest blood type in the United States is AB+ blood, at three percent.

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Donating Blood

Many people need blood transfusions. According to the Red Cross, a single car accident victim may require up to 100 pints of blood. Individuals with sickle cell anemia or cancer require frequent blood transfusions. Because of the wide variety of blood types, O- blood is important for avoiding compatibility issues with rarer blood types. Unfortunately, this high demand along with the high number of type O individuals in the world means that type O blood is often short in supply. Blood donations allow hospitals to keep blood in stock to assist as many patients as possible.

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There are many legends and beliefs about blood types. In some cultures of the world, people believe that certain blood types are more likely to have certain personality traits. Type A blood type is calm and trustworthy, type B is creative and energetic, type AB is emotional, and type O is confident. Additionally, some individuals adopt diets meant specifically for their blood. Currently, no published evidence supports that blood type has any effect on personality or that a person’s diet should change based on their blood type.

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