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External defenses: the primary barriers

These barriers try to prevent the penetration of microbes in a passive way. There are four types of external defenses (physicalmechanicalchemical and microbiological) that can work together.

Physical barriers

The physical barriers are the skin (hardened by keratin) and the mucous membranes that line the body and the cavities of the devices that communicate with the outside. The microorganisms take advantage of the breakdown of these barriers (by wounds, ulcerations, ...) to penetrate inside the organism.

The skin is the body's first defense against the entry of any microorganism. The skin is dry due to keratin, but non-keratinized skin, such as that of the mouth, nostrils, or anus, changes in appearance and is called mucosa. Mucous cells secrete mucus and therefore are moist. Mucus fixes and immobilizes many microorganisms, preventing them from penetrating.

Mechanical barriers

The mechanical barriers prevent mechanical, nonspecific passage of microorganisms. They are expulsion systems that allow the dragging of microorganisms and other foreign particles to prevent them from entering the body. For example, the cilia of the epithelial cells of the respiratory tract, whose movement eliminates microorganisms and other foreign elements existing in the mucus that covers them. Also, the flow of urine from the urinary bladder to the outside, tears, and bowel movement also favor the entrainment and expulsion of microorganisms.

Chemical barriers

Some secretions act as a chemical barrier against germs. Some examples of these types of barriers are:

  • The salivatears and nasal mucus produce the enzyme lysozyme, which destroys the bacterial cell wall. In this way, the natural openings of our body (mouth, eyes and nostrils) that lack keratin and are covered by mucous membranes, are protected.
  • The skin, in addition to preventing microorganisms from entering its cells, contains sebaceous glands that produce fatty acids and lactic acid that lower the pH, preventing many microorganisms from developing.
  • The stomach, with the hydrochloric acid from gastric juice, protects the stomach from microorganisms that can be contained in food.
  • The vaginal epithelium also produces acid secretions that prevent the growth of microorganisms.

Microbiological barriers

The autochthonous bacterial flora that lives as commensal or in symbiosis on the skin and in the digestive and urogenital systems, produces substances that prevent the proliferation of microorganisms, in addition to competing with them for nutrients.


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