Human blood is grouped into four types: A, B, AB, and O. Each letter refers to a kind of antigen, or protein, on the surface of red blood cells. For example, the surface of red blood cells in Type A blood has antigens known as A-antigens.

The Rh Factor

Each blood type is also grouped by its Rhesus factor, or Rh factor. Blood is either Rh positive (Rh+) or Rh negative (Rh-). About 85% of Americans have Rh+ blood.

Rhesus refers to another type of antigen, or protein, on the surface of red blood cells. The name Rhesus comes from Rhesus monkeys, in which the protein was first discovered.

Why Know Your Blood Type

Knowing a person's exact blood type is critical when a blood transfusion is necessary. During a blood transfusion, a patient must receive a blood type that is compatible with his or her own. If the blood types are not compatible, red blood cells will clump together, making clots that can block blood vessels and cause death.

People with Type O negative blood are considered to be universal donors because they can donate to people of any blood type. Individuals with Type AB+ blood are considered the universal recipients because people with that type can receive any blood type.

Blood Type Chart: Below is a chart listing blood types, the percentage of Americans with that type, and the types they can donate to.

Blood type Percent of Americans with this type Who can receive this type? O+ 37 O+, A+, B+, AB+ O- 6 All blood types A+ 34 A+, AB+ A- 6 A+, A-, AB+, AB- B+ 10 B+, AB+ B- 2 B+, B-, AB+, AB- AB+ 4 AB+ AB- 1 AB+, AB-

Did You Know?

Blood type is inherited, just like eye color.

Certain blood types are more common in certain countries. In China, over 99 percent of the population has Rh+ blood.

Different kinds of animals have different kinds of blood. Dogs have 4 blood types; cats have 11; cows have about 800.

Some people think blood type tells about personality. Legend has it that Type A is calm and trustworthy; Type B is creative and excitable; Type AB is thoughtful and emotional; and Type O is a confident leader.

In Japan, the idea of blood type as personality type is so popular that Japanese ask "What's your blood type?" about as often as Americans ask "What's your sign?"

Blood Transfusions

A blood transfusion is the transfer of blood from one person to another. Blood that is lost through an injury, an illness or a surgery can be replaced through transfusion. Aside from transferring blood as a whole, parts of blood, such as red blood cells, platelets or plasma can also be transferred to individuals.

Donor blood is tested for HIV, hepatitis, syphilis, West Nile virus and other diseases before transfusion. According to the Centers for Disease Control, there are more than 9.5 million blood donors in the United States and an estimated 5 million patients who receive blood annually, resulting in a total of 14.6 million transfusions per year.

The American Association of Blood Banks maintains a database of locations for individuals to donate blood. Many blood donation centers are run by the Red Cross.

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