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Blood Types

There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called antigens (also called agglutinogens ) on the surface of the RBC.

There are two antigens, A and B. If you have the A antigen on the RBC, then you have type A blood. When B antigen is present, you have type B blood. When both A and B antigens are present, you have type AB blood. When neither are present, you have type O blood.

When an antigen is present on the RBC, then the opposite antibody (also called agglutinin ) is present in the plasma. For instance, type A blood has anti-type-B antibodies. Type B blood has anti-type-A antibodies. Type AB blood has no antibodies in the plasma, and type O blood has both anti-type-A and anti-type-B antibodies in the plasma. These antibodies are not present at birth but are formed spontaneously during infancy and last throughout life.

In addition to the ABO blood group system, there is an Rh blood group system. There are many Rh antigens that can be present on the surface of the RBC. The D antigen is the most common Rh antigen. If the D antigen is present, then that blood is Rh+. If the D antigen is missing, then the blood is Rh-. In the United States , 85 percent of the population is Rh+ and 15 percent is Rh-. Unlike in the ABO system, the corresponding antibody to the Rh antigen does not develop spontaneously but only when the Rh- person is exposed to Rh antigen by blood transfusion or during pregnancy. When an Rh- mother is pregnant with an Rh+ fetus, then the mother forms antibodies that can travel through the placenta and cause a disease called hemolytic disease of the newborn (HDN), or erythroblastosis fetalis.

Donating Blood

A unit of blood is 1 pint (450 milliliters) and is mixed with chemicals (CPD) to prevent clotting. Each year, approximately 12 million to 14 million units of blood are donated in the United States . Generally, a blood donor must be at least 17 years old, be healthy, and weigh over 110 pounds.

Prior to donating blood, the donor is given an information pamphlet to read. A health history is taken to ensure that the donor has not been exposed to diseases that can be transmitted by blood, and to determine if donating blood is safe for that person's own health. The donor's temperature, pulse, blood pressure and weight are obtained. A few drops of blood are obtained to make sure the donor is not anemic. It usually takes less than 10 minutes for the blood to be removed once the needle has been placed. Sterile, single-use equipment is used so there is no danger of infection to the donor. Donors should drink extra fluids and avoid exercise that day. Blood can be donated every eight weeks.

Autologous blood donation is the donation of blood for one's own use, usually prior to surgery. Apheresis is the procedure in which only a specific component of a donor's blood is removed (usually platelets, plasma or leukocytes). In this way, more of that specific component can be removed than can be derived from one unit of blood.

Each unit of blood can be separated into several components so that each component can be given to someone with a need for that specific one. Therefore, a single unit of blood can help many people. These components include:

Packed RBCs
Fresh frozen plasma
Cryoprecipitate anti-hemolytic factor
Factor VIII concentrate
Factor IX concentrate

Let's look at each of these blood components in more detail.

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