It would be impossible for the body to stock all the cells it needs to deal with every occurrence. Instead, it keeps a relatively small supply of non-specialized precursor cells that are ready to burst into action when signaled. The specific signal, a chemical message received from other, often remote, organs or tissues, drives their specialization and determines their ultimate function. These signals trigger a proliferation of the specific cells needed to answer the demand. The chemical messengers are generally called growth factors, so-named because they are necessary to support the growth of blood cells under laboratory conditions.
Growth factors stimulate the maturation and proliferation of all blood cells.
Therapeutic use of growth factors begins when dire need outstrips the body's ability to cope, or when the body is unable to produce a necessary messenger. Since these growth factors are normally present in very small quantities, it is virtually impossible to isolate and purify commercially sufficient amounts from human plasma. This is one area where biotechnology has already contributed greatly by making it possible to produce the proteins using recombinant DNA technology. EPO triggers the production of red blood cells and if it is not produced, as is the case with long-term kidney disease, debilitating anemia can result. EPO is not only a commercial success, but it has revolutionized the lives of a majority of dialysis patients.