Blood contains different components, including red blood cells, white blood cells, platelets, and plasma. Plasma contains antibodies (immunoglobulins) and clotting factors. Not all components are produced from a particular unit of donated blood. For example, immunoglobulins and clotting factors may be prepared from plasma pooled together from many donors. White blood cells and platelets are obtained by apheresis (see Controlling Diseases by Purifying the Blood). Depending on the situation, people may receive only the red blood cells, platelets, plasma, or clotting factors. Transfusing only selected blood components allows the treatment to be specific, reduces the risks of side effects, and can efficiently use the different components from a single unit of blood to treat several people.
Sometimes blood products are treated with radiation to reduce the risk of the transfused white blood cells attacking the recipient (graft-versus-host disease—see Graft-versus-host disease).
Packed red blood cells, the most commonly transfused blood component, can restore the blood's oxygen-carrying capacity. This component may be given to a person who is bleeding or who has severe anemia. The red blood cells are separated from the fluid component of the blood (plasma) and from the other cellular components. This step concentrates the red blood cells so that they occupy less space, thus the term “packed.”
Sometimes red blood cells are specially prepared (washed) so they can be given to people who have had severe reactions to plasma. Washed red blood cells are free of almost all traces of plasma, most white blood cells, and platelets. Special filters are routinely used to remove white blood cells to reduce many types of side effects, including fever, chills, cytomegalovirus (CMV) infection, and antibody formation against human leukocyte antigens (HLA). HLA antigens are chemical markers located on the surface of cells and are unique in each organism, enabling the body to distinguish self from nonself.
Red blood cells can be refrigerated for up to 42 days. In special circumstances—for instance, to preserve a rare type of blood—red blood cells can be frozen for up to 10 years.
Platelets are small cell-like particles in the blood that help clots form. Platelets are usually given to people with too few platelets (thrombocytopenia—see Overview of Thrombocytopenia), which may result in severe and spontaneous bleeding. Platelets can be stored for only 5 days.
In the past, several donors were needed to provide enough platelets to benefit one person. Newer collection techniques that better separate platelets from other blood components allow a single donor to provide enough platelets for one person's needs.
Plasma, the fluid component of the blood, contains many proteins, including blood clotting factors. Plasma is usually frozen immediately after it is separated from fresh blood (fresh frozen plasma). Plasma frozen within 24 hours of collection can be stored for up to 1 year. It is used for bleeding disorders in which the missing clotting factor is unknown or when the specific clotting factor is not available. Plasma also is used when bleeding is caused by insufficient production of all or many of the different clotting factors as a result of liver failure or severe infection.
Blood clotting factors are proteins found in blood plasma that normally work with platelets to help the blood clot. Without clotting factors, bleeding would not stop after an injury. Certain concentrated clotting factors from plasma are called cryoprecipitate. Cryoprecipitate is most often given to people who have severe bleeding and too little fibrinogen, an important clotting factor (for example, in people with disseminated intravascular coagulation or placental abruption). Individual clotting proteins also can be purified from pooled plasma or manufactured using genetic recombinant techniques. Individual concentrated blood clotting factors can be given to people who have an inherited bleeding disorder, such as hemophilia or von Willebrand disease, and to reverse the effects of drugs that inhibit blood clotting (anticoagulants such as warfarin).
Antibodies (immunoglobulins), the disease-fighting components of blood, are sometimes given to provide temporary immunity to people who have been exposed to an infectious disease or who have low antibody levels. Infections for which antibodies are available include chickenpox, hepatitis, rabies, and tetanus. Antibodies are produced from treated plasma donations from multiple donors.
White blood cells are transfused to treat life-threatening infections in people who have a greatly reduced number of white blood cells or whose white blood cells are functioning abnormally. The use of white blood cell transfusions is rare, because improved antibiotics and the use of cytokine growth factors that stimulate people to produce more of their own white blood cells have greatly reduced the need for such transfusions. White blood cells are obtained by apheresis and can be stored for up to 24 hours.
Researchers have attempted to create blood substitutes that use certain chemicals or specially treated solutions of hemoglobin (the protein that allows red blood cells to carry oxygen) to carry and deliver oxygen to tissues. These solutions can be stored at room temperature (often for several years―much longer than blood can be kept in a blood bank) and do not need to be typed and cross-matched to the person receiving them. These characteristics make them attractive for transport to the site of trauma or to the battlefield. However, studies have not shown that any of the blood substitutes developed so far save lives. Doctors are doing further research on other possible blood substitutes.
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