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Biology 2nd Baccalaureate

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13.2.1. Nonspecific mechanisms

Fundamental insights on defense mechanisms against nonspecific infections

When a foreign substance or agent tries to invade the body, it first has to overcome primary barriers, external defenses that prevent access:

  • Physical barriers: skin and mucous membranes.
  • Mechanical barriers: cilia in the airways, urine flow, bowel movement, etc. that carry and eliminate microorganisms.
  • Chemical barriers: lysozyme (in saliva, tears, nasal mucus), fatty acids and lactic acid (sebaceous glands of the skin), gastric juice, acid secretions of the vaginal epithelium, etc. they act against the action of pathogenic microorganisms.
  • Microbiological barriers: bacterial flora on the external surface of the body or the digestive and urinary systems compete against pathogenic microorganisms.

These barriers are nonspecific, but if they are overcome, a second defensive barrier acts, nonspecific internal defenses:

  • Phagocytosis. Phagocytes are leukocytes with non-specific phagocytic capacity that, through pseudopods, encompass unusable microorganisms and cells, forming the phagosome, to later digest them in their lysosomes. There are two kinds:
    • Microphages. The most abundant.
    • Monocytes, which will transform into macrophages. Larger and with greater phagocytic capacity.
  • Inflammatory response. When a microorganism manages to cross the primary barriers, the injured cells secrete mediators of inflammation , which provoke the inflammatory response. Capillaries dilate and become more permeable, allowing phagocytes to reach and defend the body.
  • Interferon. Proteins released by virus-infected cells that prevent the spread of infection.
  • Complement system. Proteins in blood plasma that complement and enhance the action of antibodies:
    • It favors the action of phagocytes through opsonization.
    • Invasive pathogenic cell lysis.
    • Mediator of inflammation.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.