Blood Types and Distributions
There are four basic types of blood—A, B, AB, and O—each with a positive and negative sub-type. O positive is the most common type, with AB negative being the least common. The letter (A, B, AB, O) designation of the blood type indicates which antigens are present in the blood cell. The antigen determines which blood types will join if donated (see table below for which type will join with which). Types A and B have A and B antigens, respectively, type AB has both A and B antigens, and type O has no antigens—thus, it can be joined with any blood type, and is known as the universal donor.

Blood type also indicates which antibodies are present in the blood cell. The antibodies determine which blood cells body being donated to will accept. Type A contains a B antibody, type B contains an A antibody, type O contains A and B antibodies, and type AB contains no antibodies—a person with type AB blood can receive blood from any blood type.

The positive and negative classifications indicate the presence, or lack thereof, of Rhesus antigens. Blood cells that contain Rhesus antigens (Rh positive) are designated with a "+"; blood cells without Rhesus antigens (Rh negative) are designated with a "-." People with Rh positive blood can receive blood from those with Rh negative blood; people with Rh negative cannot receive Rh positive blood, as potentially life-threatening transfusion reactions can occur.

The following table shows the distribution and compatibility of each type:

Type	% of Population*	Donates to...	Receives from...
A-	6%	A-/+, AB-/+	A-, O-
A+	34%	A+, AB+	A-/+, O-/+
B-	2%	B-/+, AB-/+	B-, O-
B+	9%	B+, AB+	B-/+, O-/+
AB-	1%	AB-/+	A-, B-, AB-, O-
AB+	3%	AB+	A-/+, B-/+, AB-/+, O-/+
O-	7%	A-/+, B-/+, AB-/+	O-
O+	38%	A+, B+, AB+	O-/+

When non-compatible blood is introduced into a system, for instance, type A blood is donated to a person with type B blood, or Rh positive blood is donated to a person with Rh negative blood, several serious medical complications, known as transfusion reactions, may occur. These include fever, shortness of breath, bodily aches and pains, rapid heart rate, chills, and low blood pressure, as well as allergic reactions, which could cause itching, hives, wheezing, and, most seriously, anaphylactic shock. While transfusion reactions are alarming, they are rarely fatal if treated immediately.

Every 3 seconds someone in the United States needs blood. Donating just one pint of your blood can help save the lives of up to three people. Please consider donating blood today. (You can only donate whole blood once every eight weeks.) You can find more information on blood donation, including locations where you can donate, at the Red Cross' blood donation website, givelife.org .

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