Chemical Barriers

Enzymes in body fluids provide a chemical barrier to pathogens. Gastric juice, for example, contains the proteinsplitting enzyme pepsin and has a low pH due to the presence of hydrochloric acid. The combined effect of pepsin and hydrochloric acid kills many pathogens that enter the stomach. Similarly, tears contain the enzyme lysozyme, which has an antibacterial action against certain pathogens that may get onto the surfaces of the eyes. The accumulation of salt from perspiration also kills certain bacteria on the skin.

Interferons (in"ter-fer'onz) are hormonelike pep-tides that certain cells produce, including lymphocytes and fibroblasts. Once released from a virus-infected cell, interferon binds to receptors on uninfected cells, stimulating them to synthesize proteins that block replication of a variety of viruses. Thus, interferon's effect is nonspecific. Interferons also stimulate phagocytosis and enhance the activity of certain other cells that help to resist infections and the growth of tumors.

Other antimicrobial biochemicals are defensins and collectins. Defensins are peptides produced by neu-trophils and other types of granular white blood cells, and in the intestinal epithelium, the urogenital tract, kidneys, and the skin. Recognition of a nonself cell surface or viral particle triggers the expression of genes that encode defensins. Some defensins act by making holes in bacterial cell walls and membranes, which are sufficient to cripple the microbes. Collectins are proteins that provide broad protection against bacteria, yeasts, and some viruses. These proteins home in on slight differences in the structures and arrangements of sugars that protrude from the surfaces of pathogens. Collectins detect not only the sugar molecules, but the pattern in which they are clustered, grabbing on much like velcro clings to fabric; thus making the organism more easily phagocytized.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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