About 5 million Americans need blood transfusions every year, for all sorts of reasons. Sometimes, a transfusion is an emergency (like losing blood after an accident). Sometimes it's expected (as with treatment for cancer).

Whatever the reason, blood transfusions are one of the most common hospital procedures.

While transfusions are common, there's a lot more to them than just taking blood from one person and using it to help someone else. Each unit of blood goes through a dozen tests to check for infectious diseases and establish the blood type.

Four Blood Groups...

It might seem like blood is blood — it all looks pretty much the same to the naked eye. But although all blood contains the same basic components (red cells, white cells, platelets, and plasma), not everyone has the same types of markers on the surface of their red blood cells. These markers (also called antigens) are proteins and sugars that our bodies use to identify the blood cells as belonging in our own system.

Blood cell markers are microscopic. But they can make the difference between blood being accepted or rejected after a transfusion. So medical experts group blood into types based on the different markers.

The four main blood groups are:

1. Type A. This blood type has a marker known as "A."
2. Type B. This blood type has a marker known as "B."
3. Type AB. The blood cells in this type have both A and B markers.
4. Type O. This blood type has neither A or B markers.

Plus Rh Factor...

Some people have an additional marker, called Rh factor, in their blood. Because each of the four main blood groups (A, B, AB, and O) may or may not have Rh factor, scientists further classify blood as either "positive" (meaning it has Rh factor) or "negative" (without 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.

...Make Eight Blood Types

The different markers that can be found in blood make up 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 being 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.

Blood banks and hospitals keep careful tabs on blood type to be sure that donated blood matches the blood type of the person receiving the transfusion. Giving someone the wrong blood type can cause serious health problems.

Why Blood Type Matters

The immune system produces proteins known as antibodies that act as protectors if foreign cells enter the body. Depending on which blood type you have, your immune system will produce antibodies to react against other blood types.

If a patient is given 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 lead vital body systems — like breathing or 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 produces B antibodies. If B markers (found in Type B or AB blood) enter your body, your Type A immune system gets fired up against them. That means 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 produces 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, meaning your red blood cells have neither A or B markers, your body will have both A and B antibodies and will therefore feel the need to defend itself against A, B, and AB blood. So a person with O blood can only get a transfusion with O blood.

Type O-negative blood can be given to people with any blood type. That's because it has none of the markers that can set off a reaction. People with this blood type are considered "universal donors" and are in great demand at blood banks.

Because Type AB-positive blood has all the markers, people with this type can receive any blood type. They're called "universal recipients."

Blood transfusions are one of the most frequent lifesaving procedures hospitals perform. So there's always a need for blood donors. About 15% of blood donors are high school and college students — an impressive number when you consider you have to be 16 or 17 to donate blood.

If you'd like to help, learn more about blood donation. It's one way to be an everyday superhero and save lives!

Reviewed by: Maureen F. Edelson, MD
Date reviewed: February 2011