FractionsBackground
From Noblood
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In the early days of blood transfusion, all transfusions were of whole blood. Fresh whole blood was given to patients who had either lost a significant quantity of their own blood, or had become anemic for one reason or another. Thus, those early transfusions of blood were primarily given to replace lost volume, or to restore the patient's hemoglobin level to normal values and thereby correct tissue hypoxia. It was also felt that the presence of platelets and clotting factorsin the transfused blood would help to arrest any bleeding.
With the advent of blood banking however, medical science realized that by separating certain components from blood and administering only the ones specifically needed, the donated blood would go further and wastage could be reduced.
From then on, when a patient was anemic or it was felt that the oxygen-carrying capacity of his blood needed enhancing, he would only be given red blood cells.
A patient suffering hypovolemia or shock might be given plasma to restore normal blood volume, or for its albumin content, or because the clotting factors could address a bleeding disorder.
Platelets might be administered to a patient with ongoing bleeding, while white blood cells might be given to a patient whose immune system was struggling to combat an infection. Red cells, white cells, and platelets are often given to patients whose bone marrow function has become impaired due to chronic illness, or as a result of chemo- or radiotherapy.
The red cells, white cells and platelets are referred to by hematologists as the formed elements.
Because the formed elements and the plasma are the major constituents of blood, were the first components to be separated from whole blood, and because they play such a fundamental role in the immediate life support and maintenance of the body, many have termed them the "primary components" of blood. The process of separating blood into its various medically useful components is known as pheresis.
In the course of time, the four primary components were further processed so as to yield a number of other products, sometimes referred to as "minor blood fractions."
From plasma, for instance, comes albumin, a protein often given in conjunction with other substances as a resuscitation fluid to replace lost volume or to correct tissue edema and shock. Small amounts of albumin are sometimes added to certain drugs as a stabilizer.*
An important group of plasma proteins that are used therapeutically are immunoglobulins (or antibodies). These, as the name implies, are part of the immune system. A number of vaccines are prepared from these antibodies. Two of the better known vaccines are the anti-tetanus serum and the anti-D serum; the latter being given to mothers of childbearing age where there is an Rh-Factor issue with pregnancies. Anti-snake venom is another such serum and is usually extracted from horses' blood.
Certain clotting factors used to treat hemophilia and other bleeding disorders are also harvested from plasma, as is fibrinogen, a protein that can be mixed with other agents to make a sealant or "glue" to help repair damaged tissue. A concentrated mixture of clotting factors, fibrinogen and fibronectin, known as cryoprecipitate is sometimes given to patients with coagulopathies.
From white blood cells come interferons and interleukins. The former have proved useful in the treatment of cancer as well as some chronic infections such as hepatitis, while the latter help to impart immunity against a wide variety of diseases.
Platelets have yielded wound healing factor, a useful adjunct to the clotting system; and the patient's own platelets can also be processed and made into platelet gel, a topical agent to control bleeding.
Most recently, bovine or human red blood cells have been harvested to provide a source of free hemoglobin, the molecule that carries oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. The hemoglobin has been genetically altered in its molecular structure and encapsulated in lipid-based membranes so as to make it therapeutically useful. Three such hemoglobin-based oxygen carriers (HBOCs) currently being used in trials are Hemopure, Hemolink and Polyheme. These solutions are being heralded as the first true blood substitutes, because, when transfused into a patient they have one of the same functions as the red blood cells. They do not, however, remove carbon dioxide.
This list is not exhaustive and there will doubtless be other products in the future that come into the category of blood fractions.
It goes without saying that the collection of large amounts of blood is required to provide the raw material for the manufacturing of all these products.
- Procrit and Epogen, two brands of recombinant human erythropoietin (EPO) marketed in the US, are packaged in vials that contain very small amounts of human serum albumin. The albumin acts as a buffer between the EPO and the vial. Eprex, available in Canada and other countries, originally contained albumin, but since 2001 has been formulated without albumin. Another brand, Recormon, marketed principally in Europe and Australasia, comes in pre-filled syringes and is albumin-free. Aranesp (darbepoeitin alfa) is also presently buffered with human serum albumin.
