The distribution of blood types varies by location and genetic background, but most people around the world have Type O+ blood. Health professionals categorize blood types based on whether or not the blood contains specific antigens and antibodies. The presence or absence of A, B, and Rh(D) antigens define the eight most commonly used blood types.
What Defines Blood Types?
When closely linked genes or a single gene defines whether specific antigens are in the blood, scientists refer to those antigens as a group. Antigens are inherited sugars or proteins carried on the surface of red blood cells. The ABO and Rh blood group systems are the two most frequently used. Together these two groups define what the American Red Cross identifies as the eight most common blood types: A+, A-, B+, B-, O+, O-, AB+, and AB-.
ABO Blood Group System
The ABO blood group system categorizes blood depending on the presence or absence of A and B antigens in red blood cells. Type A blood carries only A antigen. Type B carries only B antigen. Type AB carries both antigens. Type O carries neither A nor B antigen and is the most common ABO type worldwide.
Blood plasma carries antibodies that can signal the immune system to fight antigens not native to the body. For example, people with Type A blood carry A antigen. Their blood also carries B antibody, which tells the immune system to fight off the foreign B antigen.
If a patient with blood Type A receives blood with B antigen, the patient's B antibody can bind with the B antigen causing agglutination, or clumps in the blood. Agglutination can slow or block blood circulation, cause red blood cells to rupture, and lead to kidney failure or death.
To avoid life-threatening incompatibilities, people with Type A blood should only receive blood that does not carry B antigen. As such, they should receive only Type A or Type O blood. People with Type A blood should not receive Type B or Type AB blood, because both carry B antigen.
The following chart details blood compatibility between red blood cell donors and recipients based on the ABO blood groups.
ABO Blood Group Type
Red Blood Cells Contain Antigen
Can Receive Red Blood Cells From
Can Donate Red Blood Cells To
Types A & O
Types A & AB
Types B & O
Types B & AB
Antigens A & B
All ABO Types
Antibodies A & B
All ABO Types
Rh Blood Group System
Scientists have identified fifty Rh antigens. However, when health professionals refer to blood as Rh positive or Rh negative, they are noting only the presence or absence of Rh(D) antigen. Rh(D) is the Rh antigen most likely to create a blood incompatibility.
People who are Rh negative do not possess the Rh(D) antigen, while people who do possess the Rh(D) antigen are positive. All of the ABO blood types can be either Rh negative or Rh positive. For example, AB+ (AB positive) blood contains the A, B, and Rh(D) antigens. Blood Type O- (O negative) does not contain any of these antigens.
The immune systems of people with Rh-negative blood generally do not produce Rh(D) antibody. As such, their immune systems do not naturally fight off the Rh(D) antigen found in Rh-positive blood. However, exposure of an Rh negative person to Rh(D) antigen can sensitize the immune system and cause the production of Rh(D) antibody. A person with Rh-negative blood can begin to produce Rh(D) antibody after a blood transfusion with Rh positive blood or after pregnancy with an Rh positive fetus.
Rh Type and Transfusions
Most people are Rh positive and can receive blood from Rh positive or Rh negative donors. However, people who are Rh negative should receive only Rh-negative blood.
The first exposure to Rh-positive blood is unlikely to cause a problem in an Rh-negative person. However, a second transfusion of an Rh negative patient with Rh-positive blood would likely cause Rh(D) antibody, created by the first transfusion, to fight off the Rh(D) antigen. This reaction can lead to agglutination and life-threatening complications. Additionally, giving Rh positive blood to an Rh-negative person who later becomes pregnant can cause the immune system to reject an Rh positive fetus.
Rh Type and Pregnancy
When an Rh-negative person becomes pregnant, health professionals take special precautions. First, they determine if a previous transfusion or pregnancy has exposed this person to Rh(D) antigen and caused the production of Rh(D) antibody. Exposure to Rh(D) antigen during pregnancy can occur via the placenta or as a result of trauma or fetal testing Rh(D) antibody.
If a person with Rh(D) antibody carries an Rh-positive fetus, the antibody can trigger an immune response called hemolytic disease of the fetus or newborn (HDFN). The reaction can destroy or compromise fetal/newborn blood cells, causing liver damage, severe anemia, and even stillbirth or death.
Patients known to produce Rh(D) antibody must be monitored closely during pregnancy unless the father's genetic makeup rules out an Rh-positive fetus. A person who produces Rh(D) antibody may face choices that include fetal blood type tests, fetal monitoring, fetal blood transfusions, and early delivery.
Even if people who are Rh negative and pregnant do not produce Rh(D) antibody, health professionals advise precautions to reduce the risk of developing Rh(D) antibody. Unless the father's genes rule out an Rh-positive fetus, international standards call for the administration of anti-D immune globulin. The medication is also known as anti-D IgG, Rh D IgG, and RhoGAM.
Protocols vary, but health professionals often give anti-D IgG at 28 weeks of gestation and whenever fetal blood may be present, including at the time of fetal tests, trauma, or abortion. Another dose may be given after delivery unless the baby is Rh negative.
Other Blood Groups
Although the ABO and Rh are the two central blood group systems used, other blood groups can also cause incompatibilities. Health workers also screen blood using over 30 blood group systems, including the Kell, MNS, and Lewis systems.
The World Blood Types By Percentage of Population
|Rank||Blood Type||World Population (%)|