Question

How does the body defend against invasion?

Answer 1

The human body has three lines of defense against invasion by microbes: (1) the barriers of skin and mucous membranes; (2) nonspecific internal defenses, including phagocytosis, killing by natural killer cells, inflammation, and fever; and (3) the immune response. The skin physically blocks the entry of microbes into the body. The mucous membranes of the respiratory and digestive tracts secrete antibiotic substances, antibodies, and mucus that traps microbes. If microbes do enter the body, white blood cells travel to the site ofentry and engulfthe invading cells. Natural killer cells secrete proteins that kill infected or cancerous cells. Injuries stimulate the inflammatory response, in which chemicals are released that attract phagocytic white blood cells, increase blood flow, and make capillaries leaky. Later, blood clots wall off the injury site. Fever is caused by endogenous pyrogens, chemicals released by white blood cells in response to infection. High temperatures inhibit bacterial growth and accelerate the immune response.