Serum is the straw colored liquid part of blood from which blood cells and the chemicals which are causing clotting have been taken out. If only blood cells are removed from the blood, the liquid remaining is plasma. Plasma will clot but serum does not clot.
Serum carries proteins and chemicals such as calcium, sodium, potassium, chlorides and sugar in solution. These and other substances are necessary for proper functioning of the body. The proteins in serum are called albumin and globulin. By the process called osmosis, the albumin keeps a balance between the fluid inside the tissue cells and the fluid in the blood stream. If the albumin runs low in the blood, osmotic pressure will pull water into the tissues which then swell and produce a condition called edema. This may occur in certain cases of nephritis of kidney disease. It is a frequent occurrence in severe burns and where there is loss of blood and shock. A concentrated solution of albumin is given by transfusion in these cases.
By a process similar to electrolysis, the globulin protein can be separated into three main fractions. These are called alpha, beta, and gamma. Gamma globulin contains the antibodies of the body. These substances neutralize certain diseases and produce immunity to other diseases.
People who have recovered from a deadly disease such as tetanus have antibodies in their blood. Antitetanus serum made from their blood can be injected into another person who has contracted tetanus to fight the disease.