Nonspecific Mechanisms of Defense

The body possesses many mechanisms that impart nonspecific defense. The objectives of these mechanisms are to prevent microorganisms from gaining a foothold in the body and to destroy them if they penetrate to the deeper tissues.

Mechanical barriers. Mechanical barriers at the portal of entry represent the first line of defense for the body. These defenses are normally part of the body's anatomy and physiology. The skin is a representative example. The outermost layers of skin consist of compacted, cemented cells impregnated with the insoluble protein keratin. The thick top layer is impervious to infection and water. In the unbroken state, it usually is not penetrated by pathogens.

The mucous membranes of the urinary, respiratory, and digestive tracts are another example. They are moist and permeable, but their fluids, such as tears, mucus, and saliva, rid the membrane of irritants. Nasal hairs trap particles in the respiratory tract, and the fluids exert a flushing action. Cilia on the cells sweep and trap particles in the respiratory tract, and coughing ejects irritants.

Chemical defenses. Among the nonspecific chemical defenses of the body are the secretions of lubricating glands. The tears and saliva contain the enzyme lysozyme, which breaks down the peptidoglycan of the cell wall of Gram-positive bacteria. Thelactic acid of the vagina imparts defense, and the extremely caustic hydrochloric acidof the stomach is a barrier to the intestine. Semen contains the antimicrobial substancespermine that inhibits bacteria in the male urogenital tract.

Genetic barriers. The hereditary characteristics of an individual are a deterrent to disease as well. For example, humans suffer HIV infection because their T-lymphocytes have the receptor sites for the human immunodeficiency virus. Dogs, cats, and other animals are immune to this disease because they do not possess the genes for producing the receptor sites. Conversely, humans do not suffer canine distemper because humans lack the appropriate receptor sites for the virus that causes the disease.

Inflammation. Inflammation is a nonspecific response to any trauma occurring to tissues. It is accompanied by signs and symptoms that include heat, swelling, redness, and pain. Inflammation mobilizes components of the immune system, sets into motion repair mechanisms, and encourages phagocytes to come to the area and destroy any microorganisms present.

Inflammation can be controlled by nervous stimulation and chemical substances calledcytokines. These chemical products of tissue cells and blood cells are responsible for many of the actions of inflammation. The loss of fluid leads to a local swelling callededema. In some types of inflammation, phagocytes accumulate in the whitish mass of cells, bacteria, and debris called pus.

Fever. Fever is considered a nonspecific defense mechanism because it develops in response to numerous traumas. Fever is initiated by circulating substances calledpyrogens, which affect the brain's hypothalamus and cause the latter to raise the temperature. Although excessive fever can be dangerous, fever is believed to have a beneficial role because it retards the growth of temperature-sensitive microorganisms (for example, leprosy bacilli), and it increases the metabolism of body cells while stimulating the immune reaction and the process of phagocytosis.

Interferon. Interferon is a group of antiviral substances produced by body cells in response to the presence of viruses. Lymphocytes and macrophages produce alpha-interferon, epithelial cells produce beta-interferon, and T-lymphocytes producegamma-interferon. The interferons do not directly inhibit viruses. Instead, they stimulate adjacent cells to produce substances that inhibit the replication of viruses in those cells. Interferons produced in response to one virus will protect against many other types of viruses, and for this reason, interferon is considered a nonspecific form of defense.