The As, Bs and Os of Blood Types

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By News Team on February 10, 2020

Do you know your blood type? And do you know why it matters?

Every two seconds, someone needs a blood transfusion in the United States. And every year, those transfusions save 4.5 million lives.
But all donated blood doesn't work for all people. There are four major blood groups, and within those groups, there are eight basic blood types.

“Getting a safe transfusion requires careful matching of blood types and can determine whether someone lives or dies,” says Carilion Clinic's Chair of Pediatrics, Kimberly Dunsmore, M.D.

Blood Type Characteristics

“Your blood group is inherited and is based on the type of antigens and antibodies you have,” says Dr. Dunsmore. “Certain antigens can cause your immune system to attack any transfused blood that is incompatible.”

An antigen is a protein molecule found on the surface of many red blood cells and can prompt an immune response. An antibody is a protein created in the body to fight antigens such as viruses and bacteria but also other foreign proteins such as incompatible blood.

The four major blood groups—known as the ABO Blood Group System—are:

  • Group A, which contains the A antigen and B antibody
  • Group B, which has the B antigen and the A antibody
  • Group AB, which has both A and B antigens but no A or B antibodies
  • Group O, which has no A or B antigens but has both A and B antibodies

A third type of antigen, the Rh factor, is a protein that determines whether your blood type is “positive” (Rh+) or “negative” (Rh-).

The Rh factor creates the full eight blood types, since the groups A, B, AB and O can be either positive or negative.

In addition, 600 other known antigens may occur in the blood and create rare blood types, according to the American Red Cross.

Donating Blood

Type O is the most common blood type and is in demand by hospitals and blood banks. Type O-negative blood is sometimes referred to as “the universal blood type" because it is used for emergency transfusions and for immune-deficient infants.

There can be risks even with O-negative blood, however, and donors of each blood type are urged to give blood.

“We prefer to get an exact match between a recipient and a donor to avoid potential problems with the body rejecting transfused blood,” says Dr. Dunsmore.
A-positive is another common blood type that blood banks seek. One-third of the U.S. population has this blood type.
According to the Red Cross:

  • Group A can donate red blood cells to As and ABs
  • Group B can donate red blood cells to Bs and ABs
  • Group AB can donate to other ABs and can receive red blood cells from all other types

Type-Related Health Risks

“Most health risks from blood types occur with Rh negative women who have children with Rh positive men,” says Dr. Dunsmore.

“The mother then makes antibodies against the Rh factor, which crosses the placenta and attacks the red blood cells of the fetuses, which have Rh-positive blood. This can result in life-threatening anemia for the fetus/infant.”

Carilion Clinic also uses donor blood to treat children and adults who have severe anemia from conditions such as trauma, cancer and blood disorders like sickle cell anemia and thalassemias.  

What is important to remember: Blood donations save lives and are given every day in our hospitals.

“Be a hero and donate,” says Dr. Dunsmore. The life you save could be that of someone you love.

Where to Donate

Carilion Clinic sponsors regular blood drives; visit our calendar to find one near you. You can also donate directly at the American Red Cross.