An individual's blood type depends on which genes were passed on by their mother or father.
The best-known way of grouping of blood types is the ABO system, although there are other groups.
Within the ABO group, four major categories are divided into eight common blood types: A, B, O, and AB.
Over 9.5 million people in the United States (U.S.) are blood donors, and around 5 million patients receive blood each year, according to the Centers for Disease Control and Prevention (CDC).
It is crucial to give a patient the right blood type in a transfusion. The wrong type can trigger an adverse and potentially fatal reaction.
What makes a blood type?
The ABO system is the most familiar way of grouping human blood types.
Blood consists of cells and a yellow watery liquid known as plasma. The blood group depends on what each part of the blood contains.
The two main blood group systems are ABO antigens and Rhesus antigens (including RhD antigen). These two antigens are used to classify blood types.
Bacteria and viruses normally carry an antigen. During an infection, their antigen marks them as something that is foreign to the body or not usually found in the body.
Most red blood cell antigens are protein molecules found on the surface of red blood cells.
White blood cells produce antibodies as an immune defense. These antibodies will target antigens and attack the foreign object, for example, the bacteria.
Cross-matching of blood types is vital. If a person receives red blood cells with antigens not normally present in their system, their body will reject and attack the new red blood cells.
This can cause a severe, and possibly life-threatening reaction.
ABO and the most common blood types
The ABO blood group system is used to determine the different types of antigens in the red blood cells and antibodies in the plasma.
This system and RhD antigen status determine which blood type or types will match for a safe red blood cell transfusion.
There are four ABO groups:
Group A: The surface of the red blood cells contains A antigen, and the plasma has anti-B antibody that would attack any foreign B antigen containing red blood cells.
Group B: The surface of the red blood cells contains B antigen, and the plasma has anti-A antibody that would attack any foreign A antigen containing red blood cells.
Group AB: The red blood cells have both A and B antigens, but the plasma does not contain anti-A/anti-B antibodies. Individuals with type AB can receive any ABO blood type.
Group O: The plasma contains both types of anti-A/anti-B antibodies, but the surface of the red blood cells does not contain any A/B antigens. Having none of these A/B antigens means that they can be donated to a person with any ABO blood type.
Some red blood cells have the Rh factor, which is also called RhD antigen.
Rhesus grouping adds another dimension.
If the red blood cells contain the RhD antigen, they are RhD positive. If they do not, they are RhD negative.
This means that there are eight main blood types in the ABO/RhD blood group system. Some of these are more common than others.
The eight main blood types are A, B, O, or AB, and each type can be positive or negative.
- A-positive (A+) occurs in 30 percent of people in the U.S.
- A-negative (A-) occurs in 6 percent of people
- B-positive (B+) occurs in 9 percent of people
- B-negative (B-) occurs in 2 percent of people
- AB-positive (AB+) occurs in 4 percent of people
- AB-negative (AB-) occurs in 1 percent of people
- O-positive (O+) occurs in 39 percent of people
- O-negative (O-) occurs in 9 percent of people
Around 82 percent of the population in the U.S. has RhD-positive blood. The rarest blood type is AB negative.
Universal donor and universal recipient
O negative blood contains no A or B or RhD antigens. These red blood cells can be transfused to nearly all patients of any blood type. Group O negative is known as the "universal donor" type.
AB positive blood, on the other hand, contains no anti-A/anti-B/RhD antibodies, so patients with this blood type can, therefore, receive nearly any type of red blood cell transfusion. This type is, therefore, referred to as the "universal recipient" type.
When is blood group important?
It is important to confirm a person's blood type when they are donating blood or receiving a transfusion.
If someone with group B antigen receives red blood cells from someone with group A antigen, their body will reject the transfusion.
This is because patients with B antigen on their red blood cells have anti-A antibody in their plasma. The anti-A antibody in the plasma then attacks and destroys the A antigen donor red blood cells. This can be fatal.
Blood types in pregnancy
During pregnancy, a mother may have a different RhD type to her fetus, as a fetus can inherit a different blood type from the father's genes. There is some risk involved if the mom is RhD negative and the fetus is RhD positive.
Blood testing is important in pregnancy and essential when donating or receiving a transfusion.
A small number of red blood cells from the fetus' circulation can enter the mother's bloodstream. This can lead to the creation of anti-RhD antibody in the plasma by the mother. This process is known as sensitization.
A problem can arise if this antibody then detects "foreign" antigen in the fetus' blood cells, and if they then start to attack the fetus' red blood cells as a defense mechanism.
This can lead to severe jaundice and brain damage if undetected.
An injection of anti-D immunoglobin G helps to prevent the mother's own production of this antibody and reduce the impact of a sensitizing event on the fetus.
Anti-D IgG dosing is usually given at 28 weeks and sometimes additionally at 34 weeks of pregnancy. The effect of Anti-D lasts up to 12 weeks.
If a sensitizing event causes the mother to form anti-Rh antibody, her own antibody will remain in the plasma even if anti-D igG is administered.
If the RhD-negative woman becomes pregnant again, and if the next fetus also has an RhD-positive blood type, there is higher risk that the mother's antibody will attack the red blood cells of this next fetus.
Blood testing can detect possible risks to fetal health in advance, by checking whether the fetus' blood type is compatible with the mother's.
It is also important to know the mother's blood type in case there is blood loss during delivery, as she may need a red blood cell transfusion.
Testing for blood type
A blood test can determine an individual's blood type.
Blood testing is essential for patient safety.
A technician mixes the individual's blood with a variety of serum samples. The blood type of each serum sample is already known.
Each sample consists of a different blood type, with the clotting agent removed. This is serum.
The technician will monitor how the person's blood reacts with each serum. The antibodies in the serum will cause a different reaction in each one.
In this way, the blood type can be identified.
For example, if a reaction occurs when the individual's blood is mixed with serum consisting of blood type A, which contains anti-B antibody, the unknown blood type, which is the individual's, must be type B.
Blood typing must be tested in this way before carrying out a red blood cell transfusion.
This is important because, apart from the eight main groups, are there many lesser-known blood types.
It is rarer for these other antigens to cause transfusion reactions, but it can happen, so precautions are essential.