Serum
Serum, clear, yellowish fluid that remains as the liquid portion of blood after clotting has taken place. Before clotting occurs, the liquid of the blood is called plasma . Both plasma and serum contain proteins, salts, sugars, waste products, vitamins, minerals, fats, and hormones. But plasma also contains the protein fibrinogen and certain other elements necessary for clotting, and it therefore clots as easily as whole blood. Serum, which lacks these elements, does not clot. In order to isolate serum for chemical tests and other medical uses, whole blood is placed in a glass tube and allowed to clot. The clotted blood is then placed in a centrifuge, a machine that spins the blood at high speeds, causing the clot and the cellular parts of the blood to separate from the serum and sink to the bottom of the tube. The serum remains at the top of the tube. Serum from animals like horses and sheep can be used to provide human beings with protection against infections or poisons. Animals ar