Antibodies

Antibody, any of perhaps a million kinds of normally occurring protein molecules that are produced in the body of cells called lymphocytes and that act primarily as a defense against invasion by foreign substances. An important component of the immune system, antibodies are found in the blood of all vertebrates, in the fraction of the blood called gamma globulin.

The synthesis, or manufacture, of antibodies is initiated when a foreign substance, referred to as an antigen, enters the body. Lymphocyte cells respond to the foreign substance by making an antibody with a molecular arrangement that fits the shape of molecules on the surface of the substance so that the antibody combines with it. Common antigens are the protein components of bacteria and viruses. These antigens may enter the body during infection or may be deliberately introduced by vaccination (see Immunization) in order to stimulate the production of antibodies. The binding of antibodies to the surfaces of bacteria, viruses, or toxins (see Toxin) can neutralize and eliminate these harmful substances in any or all of three ways: (1) by directly inactivating them, (2) by enabling other blood cells to engulf and destroy them (see Phagocytosis), and/or (3) by weakening their surfaces and rendering them vulnerable to destruction by other blood proteins (collectively called complement). Animals do not have antibodies to substances to which they have not been exposed, but one animal is able to produce enough different kinds of antibodies to fit the molecular arrangement of any foreign substance it is likely to encounter.

In diseases such as multiple sclerosis and systemic lupus erythematosus, the body mistakenly makes antibodies against normal tissue components (see Autoimmune Diseases). Sometimes viruses may disturb the immune mechanism.

Globular proteins

Unlike fibrous proteins, globular proteins are spherical and highly soluble. They play a dynamic role in body metabolism. Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and protein hormones. The albumins and globulins are classes of soluble proteins abundant in animal cells, blood serum, milk, and eggs. Hemoglobin is a respiratory protein that carries oxygen throughout the body and is responsible for the bright red color of red blood cells. More than 100 different human hemoglobins have been discovered, among which is hemoglobin S, the cause of sickle-cell anemia, a hereditary disease suffered mainly by blacks.

A Enzymes

All of the enzymes are globular proteins that combine rapidly with other substances, called substrate, to catalyze the numerous chemical reactions in the body. Chiefly responsible for metabolism and its regulation, these molecules have catalytic sites on which substrate fits in a lock-and-key manner to trigger and control metabolism throughout the body.

B Protein Hormones

These proteins, which come from the endocrine glands, do not act as enzymes. Instead they stimulate target organs that in turn initiate and control important activities—for example, the rate of metabolism and the production of digestive enzymes and milk. Insulin, secreted by the islands of Langerhans, regulates carbohydrate metabolism by controlling blood glucose levels. Thyroglobulin, from the thyroid gland, regulates overall metabolism; calcitonin, also from the thyroid, lowers blood calcium levels. Angiogenin, a protein structurally determined in the mid-1980s, directly induces the growth of blood vessels in tissues.

C Antibodies

Also called immunoglobulins, antibodies make up the thousands of different proteins that are generated in the blood serum in reaction to antigens (body-invading substances or organisms). A single antigen may elicit the production of many antibodies, which combine with different sites on the antigen molecule, neutralize it, and cause it to precipitate from the blood.

D Microtubules

Globular proteins can also assemble into minute, hollow tubes that serve both to structure cells and to conduct substances from one part of a cell to another. Each of these microtubules, as they are called, is made up of two types of nearly spherical protein molecules that pair and join onto the growing end of the microtubule, adding on length as required. Microtubules also make up the inner structure of cilia, the hairlike appendages by which some microorganisms propel themselves.