Page 687 - Veterinary Immunology, 10th Edition

VetBooks.ir Immunity on Body Surfaces

The Defenses of the Skin

The skin is the first line of defense against many microbial invaders.
It carries out its barrier function effectively, and few bacteria can
penetrate intact skin unaided. Skin forms a tough physical barrier

supplemented by continuous desquamation, desiccation, and a low
pH because of fatty acids in sebum. In addition, the skin carries a
resident microbiota that excludes pathogenic bacteria and fungi. If
the skin microbiota is disturbed, its protective properties are
reduced, and microbial invasion may result. Thus skin infections

tend to occur in areas such as the axilla or groin, where both pH
and humidity are high. Similarly, animals forced to stand in water
or mud show an increased frequency of foot infections; as the skin

becomes sodden, its structure breaks down and its resident
microbiota changes in response to alterations in the environment.
Major anatomical and immunological differences exist between the
thick, hairless skin of humans and the haired skin of most
mammals. Hair itself prevents desiccation and may protect against

some fungal infections as well as being important in waterproofing
and insulation.
Each layer of the skin has its own defensive mechanisms. For

example, keratinocytes express multiple PRRs such as the toll-like
receptors (TLRs), mannose receptors, and C-type lectins, so they are
well able to recognize PAMPs associated with microbial invasion.
Upon stimulation, keratinocytes produce a complex mixture of
interleukins, interferons, chemokines, and other cytokines, growth

factors, and antimicrobial proteins, all of which assist in excluding
microbes seeking to penetrate the skin. Keratinocytes are the main
source of cathelicidins and β-defensins. Pigs, for example, have 11

different skin cathelicidins. Calprotectin is also produced in the
skin. It is a metal chelator and restricts the availability of the
essential trace elements Zn and Mn to bacteria. Mast cells and skin
secretory cells such as sweat, apocrine, and sebaceous glands also
add antimicrobial peptides and lipids. Under normal conditions,

keratinocyte precursor cells divide and continually renew the

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