Blood Types and How They Affect Your Health

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Top view of assorted blood sample tubes

Human blood can be categorized into eight different types: A+, A-, B+, B-, O+, O-, AB+, and AB-. But what do these blood types mean? And how can they affect your health? Learn more about why it’s important to know your blood type, how the blood type Rh factor can affect a pregnant woman or her unborn child, and what “universal donor” means.

Blood Type Basics

Your blood consists of two main components: cells (solids) and plasma (liquids). The solids portion of your blood contains red cells, white cells, and platelets. For blood typing, only the red cells matter.

Your blood type is determined based on which antigens (proteins that activate an immune response) appear on the red blood cells. Red cells may have either A or B (or neither, or both) antigens on their surface.

All of your blood’s red cells will fall into just one of these categories:

  • Type A blood: Red cells exhibit the A antigen.
  • Type B blood: Red cells exhibit the B antigen.
  • Type AB blood: Red cells exhibit both A and B antigens.
  • Type O blood: Red cells do not exhibit either the A or B antigens.

Rh Factor: The Positive and Negative of Blood Typing

Once a lab test has established your blood type, further testing on the blood sample will reveal if your blood also contains a protein called Rhesus factor, or Rh factor. If it does, then the lab will add a positive mark to your blood typing results. If you do not possess the Rh factor, then your blood type will be listed with a negative sign.

The vast majority of people are Rh-positive, so they have a positive blood type. The eight possible blood type combinations are:

  • Type A+: About one-third of Americans have this blood type.
  • Type A-: Fewer than 10% of Americans have this blood type.
  • Type B+: Roughly 15% of the population has this blood type.
  • Type B-: Only about 1% of people have this blood type.
  • Type AB+: About 4% of the population has this blood type, and these people are considered “universal recipients” because they can safely receive any of the eight blood types.
  • Type AB-: The rarest of all blood types, fewer than 0.5% of people have this type.
  • Type O+: The most common type, almost half of all Americans are O+.
  • Type O-: Only about 4% of the population has O- blood, and these people are considered “universal donors” because O- blood can be given to anyone.

Why Blood Type Matters

It’s crucial to know your blood type in case you ever need a blood transfusion. Doctors will always check your type before giving you blood, but knowing your type can help you make sure you’re receiving the right type of transfusion. If you receive the wrong type of blood, your body’s immune system could attack it and destroy the infused red cells. Here’s why.

In addition to carrying (or not) antigens on the red cells, your blood also may contain specific antibodies (immune system cells and proteins) in the plasma that attack foreign blood antigens. For example, people with Type A blood not only exhibit the A antigen on the red cells but also a B antibody in their plasma. Those B antibodies will attack and destroy any cells they encounter that exhibit the B antigen—such as red blood cells from a Type B blood transfusion. If a person with Type A blood received a Type B transfusion, it could create a catastrophic immune system response.

Because people, in general, must receive transfusions that contain their specific blood type, blood banks constantly seek donors with all blood types. Blood banks particularly value donations from people with O- blood because it contains neither the A and B antigens nor the Rh factor, which means O- blood can be given to anyone. People with O- blood are called “universal donors.”

It’s also important for people with rare blood types to donate blood. The rare blood types include B-, AB+, and AB-. Donating blood helps ensure that others with these blood types can receive the best transfusion match.

Pregnancy and the Rh Factor

Another reason to know your blood type is so you can enjoy a healthy pregnancy. A baby can inherit the Rh factor from just one parent, which means the Rh type of the fetus won’t necessarily match the Rh type of the mother.

If a pregnant woman is Rh- and her fetus is Rh+, this Rh incompatibility can have serious health consequences. Again, because of antibodies, if the blood of an Rh+ fetus gets into the bloodstream of an Rh- mother, the mother’s body will view the Rh+ cells as a threat, attacking and destroying them. This immune system process can cause grave illness and even death for either the mother or the fetus.

Fortunately, Rh- women can receive medication that stops the immune system from viewing Rh+ cells as a threat. This safeguards the health of both mom and baby.

Knowing your blood type may seem like just a fun bit of trivia, but it’s important for keeping you safe in the event you need a blood transfusion or want to ensure a healthy pregnancy. You can find out your blood type simply by donating blood.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Dec 1
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. The Rh Factor: How It Can Affect Your Pregnancy. American College of Obstetricians and Gynecologists.
  2. Blood. U.S. MedlinePlus, National Library of Medicine.
  3. Blood Typing. MedlinePlus, U.S. National Library of Medicine.
  4. Blood Types Explained. Red Cross.
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