Nonspecific Defenses page 659

1. Species resistance

Each species is resistant to certain diseases that may affect other species but is susceptible to diseases other species may resist.

2. Mechanical barriers a. Mechanical barriers include skin and mucous membranes.

b. Intact mechanical barriers prevent entrance of some pathogens.

3. Chemical barriers a. Enzymes in gastric juice and tears kill some pathogens.

b. Low pH in the stomach prevents growth of some bacteria.

c. High salt concentration in perspiration kills some bacteria.

d. Interferons stimulate uninfected cells to synthesize antiviral proteins that block proliferation of viruses; stimulate phagocytosis; and enhance activity of cells that help resist infections and stifle tumor growth.

e. Defensins make holes in bacterial cell walls and membranes.

f. Collectins provide broad protection against a wide variety of microbes by grabbing onto them.

4. Fever a. Viral or bacterial infection stimulates certain lymphocytes to secrete IL-1, which temporarily raises body temperature.

b. Physical factors, such as heat or ultraviolet light, or chemical factors, such as acids or bases, can cause fever.

c. Elevated body temperature and the resulting decrease in blood iron level and increased phagocytic activity hamper infection.

5. Inflammation a. Inflammation is a tissue response to damage, injury, or infection.

b. The response includes localized redness, swelling, heat, and pain.

c. Chemicals released by damaged tissues attract white blood cells to the site.

d. Clotting may occur in body fluids that accumulate in affected tissues.

e. Connective tissue containing many fibers may form a sac around the injured tissue and thus aid in preventing the spread of pathogens.

6. Phagocytosis a. The most active phagocytes in blood are neutrophils and monocytes; monocytes give rise to macrophages, which remain fixed in tissues.

b. Phagocytic cells associated with the linings of blood vessels in the red bone marrow, liver, spleen, and lymph nodes constitute the mononuclear phagocytic system.

c. Phagocytes remove foreign particles from tissues and body fluids.

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