Nonspecific defenses include physical and chemical barriers, the inflammatory response, and interferons. Physical barriers include the intact skin and mucous membranes. These barriers are aided by various antimicrobial chemicals in tissue and fluids. An example of such a substance is lysozyme, an enzyme present in tears that destroys the cell membranes of certain bacteria.Inflammatory Response
Another line of defense is the inflammatory response, in which white blood cells called monocytes and granulocytes (e.g., basophils and neutrophils) reach an injured area. Basophils release histamine, which results in increased local blood flow and increased permeability of the capillaries and allows phagocytizing cells, such as neutrophils and monocytes (macrophages), into the area. The same response sometimes results in fever. Leakage of the clotting protein fibrinogen and other substances into the injured area results in blockage of tissue by clots, which wall off the injured area to retard the spread of bacteria or their toxins.
Interferons are proteins released by a virus-invaded cell that prompt surrounding cells to produce enzymes that interfere with viral replication. They are the reason that, in most instances, infection with one virus precludes infection by a second virus.
Nonsusceptibility is the inability of certain disease-carrying organisms to grow in a particular host species. Nonsusceptibility may be caused by such conditions as lack of availability of particular growth substances needed by the infecting microorganism or body temperature unsuitable for the invading microorganism. For example, chickens are nonsusceptible to anthrax because the bacteria cannot grow at the body temperature normal for that animal.
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