Section 1

Nonspecific Defenses

• A pathogen is any agent that causes a disease. Robert Koch developed four basic steps, or postulates, for identifying the pathogen responsible for a disease.

• The skin and mucous membranes are nonspecific defenses that keep pathogens out of the body.

• The skin acts as an external barrier to pathogens and also releases substances that are toxic to pathogens.

• The mucous membranes protect the interior surfaces of the body and secrete mucus, a sticky fluid that traps pathogens.

Vocabulary infectious disease (p. 957) pathogen (p. 957) Koch's postulates (p. 957) mucous membrane (p. 958)

inflammatory response (p. 959) histamine (p. 959) phagocyte (p. 959)

• Injury to cells triggers an inflammatory response. Injured cells release chemical messengers that attract phagocytes through the capillary walls. Phagocytes then destroy the pathogens.

• White blood cells fight pathogens. Two types of phagocytes (neutrophils and macrophages) ingest pathogens. Natural killer cells pierce the cell membranes of infected cells.

• Nonspecific defenses also include an elevation in temperature (fever) and the activation of proteins such as the complement system and interferon.

neutrophil (p. 960) macrophage (p. 960) natural killer cell (p. 960)

complement system (p. 960) interferon (p. 960)

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