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Blood transfusions are a lifesaving treatment for many Americans. Blood transfusions are needed for many reasons, including surgery, after accidents, and for patients with illnesses and cancer. Every 2 seconds, someone needs a blood transfusion.

Blood cannot be artificially made, so doctors rely on volunteer donations. To keep the blood supply safe, every donation is tested for blood type and checked for infectious diseases.

What Are the Components of Blood?

All blood contains these basic components:

  • red blood cells that deliver oxygen 
  • white blood cells that fight infections
  • platelets that help blood clot
  • plasma, the liquid part of blood

But not everyone has the same blood type.

What Are the Blood Types?

Categorizing blood according to type helps prevent reactions when someone gets a blood transfusion. Red blood cells have markers on their surface that characterize the cell type. These markers (also called antigens) are proteins and sugars that our bodies use to identify the blood cells as belonging in us.

The two main blood groups are ABO and Rh.

The ABO blood system has four main types:

  • Type A: This blood type has a marker known as A.
  • Type B: This blood type has a marker known as B.
  • Type AB: This blood type has both A and B markers.
  • Type O: This blood type has neither A nor B markers.

Blood is further classified as being either "Rh positive" (meaning it has Rh factor) or "Rh negative" (without Rh factor).

So, there are eight possible blood types:

  1. O negative. This blood type doesn't have A or B markers, and it doesn't have Rh factor.
  2. O positive. This blood type doesn't have A or B markers, but it does have Rh factor. O positive blood is one of the two most common blood types (the other is A positive).
  3. A negative. This blood type has A marker only.
  4. A positive. This blood type has A marker and Rh factor, but not B marker. Along with O positive, it's one of the two most common blood types.
  5. B negative. This blood type has B marker only.
  6. B positive. This blood type has B marker and Rh factor, but not A marker.
  7. AB negative. This blood type has A and B markers, but not Rh factor.
  8. AB positive. This blood type has all three types of markers — A, B, and Rh factor.

Having any of these markers (or none of them) doesn't make a person's blood any healthier or stronger. It's just a genetic difference, like having green eyes instead of blue or straight hair instead of curly.

Why Are Blood Types Important?

The immune system is the body's protection against invaders. It can identify antigens as self or nonself. To get a blood transfusion safely, a person's immune system must recognize the donor cells as a match to his or her own cells. If a match isn't recognized, the cells are rejected.

The immune system makes proteins called antibodies that act as protectors if foreign cells enter the body. Depending on which blood type a person has, the immune system will make antibodies to react against other blood types.

If a patient gets the wrong blood type, the antibodies immediately set out to destroy the invading cells. This aggressive, whole-body response can give someone a fever, chills, and low blood pressure. It can even cause vital body systems — like breathing or the kidneys — to fail.

Here's an example of how the blood type-antibody process works:

  • Let's say you have type A blood. Because your blood contains the A marker, it makes B antibodies.
  • If B markers (found in type B or type AB blood) enter your body, your type A immune system gets fired up against them.
  • This means that you can only get a transfusion from someone with A or O blood, not from someone with B or AB blood.

In the same way, if you have the B marker, your body makes A antibodies. So as a person with type B blood, you could get a transfusion from someone with B or O blood, but not A or AB.

Things are a little different for people with type AB or type O blood:

  • If you have both A and B markers on the surface of your cells (type AB blood), your body does not need to fight the presence of either.
  • This means that someone with AB blood can get a transfusion from someone with A, B, AB, or O blood.

But if you have type O blood, your red blood cells have no A or B markers. So:

  • Your body will have both A and B antibodies and will therefore feel the need to defend itself against A, B, and AB blood.
  • A person with O blood can only get a transfusion with O blood.

Blood transfusions are one of the most frequent lifesaving procedures hospitals do. Blood is collected from healthy donors every day, and more blood donors are always needed. One blood donation can save up to three lives.

Medically reviewed by: Maureen F. Edelson, MD
Date reviewed: May 2022