Blood plasma from which the protein
fibrinogen (a clotting factor) has been removed is known as serum. The most important
components of serum are the serum globulins or immunoglobulins, which are proteins associated with the body's immune response. Serum also contains various salts, glucose, amino acids, and hormones. In order to produce serum, blood is allowed to clot, and then the clear, straw-colored liquid is extracted. The clinical method of obtaining serum is by centrifugation of the blood plasma, separating the
fibrinogen from the plasma.
Because serum is easy to keep sterile and to package, it was used until recently as a medium to contain antibodies formed in the blood of animals exposed to certain
diseases. These sera were injected into human patients who suffered from the same diseases, and who required added immune defense; a side effect of these sera was sensitivity to the serum. The use of serum was significant in the elimination of many infectious diseases until it was replaced by antibiotic drugs. Human plasma transfusions still are given to people suffering from shock or severe burns to replace lost fluid.