The skin barrier is made up of three closely packed cell layers: the epidermis, dermis, and hypodermis (upper to bottom layer). The epidermis  consists of keratin, which provides mechanical strength to the skin surface, preventing enzymatic degradation. The acidic environment of the surface also prevents bacterial degradation and inhibits microbial growth. Skin shedding also sheds microbes on the skin surface. Damage to the skin barrier allows for pathogens to enter into deeper tissue.

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Innate Immune Physical Barrier: The Skin Barrier

Membranes lining the nose, mouth, lungs, and urinary and digestive tracts consisting of epithelial cells - bound together by tight junctions - that secrete mucus. The mechanical movement of mucus moves microbes away from potential sites of infection.

Innate Immune Physical Barrier: Mucous Membranes

Epithelial cells that line the blood and lymphatic vessels, urogenital tract, and other tissues which defends against invading microbes.

Innate Immune Physical Barrier: Endothelia

Types of physical barriers in innate immune physical defenses include:
- The skin barrier
- Mucous membranes
- Endothelial cells

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Types of physical defenses in the innate immune response include:
- Physical barriers
- Mechanical defenses
- Microbiome

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Types of Innate Immune Physical Defenses

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Physical Defenses of Nonspecific Innate Immunity (Table 17.2). Reprinted from OpenStax Microbiology (p. 691), by N. Parker, M. Shneegurt, A.-h. Thi Tu, B. M. Forester, and P. Lister, 2016, Rice University. Copyright 2016 by Rice University.

Physical Defenses of Nonspecific Innate Immunity Table

Innate Immune Physical Defenses: Types of Physical Barriers

Types of mechanical defenses in innate immune physical defenses include:
- Skin shedding
- Mucociliary escalator
- Peristalsis
- Flushing action of urine and tears

Innate Immune Physical Defenses: Types of Mechanical Defenses

Resident microbiota can serve to protect the body against harmful pathogens, such as by competing with opportunistic pathogens (taking up binding sites and competing for resources) prevents pathogen attachment and proliferation. They can also contribute to chemical defenses. The disruption of microbiota can increase susceptibility to infection.

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