Page 688 - Veterinary Immunology, 10th Edition

epidermis in a coordinated manner. If the skin is wounded or
VetBooks.ir inflamed, alterations in adhesion molecules, surface receptors, and

the cytokine environment change the behavior of the keratinocytes.
Keratinocytes also express MHC class II and can act as antigen-

presenting cells.
Once they invade the skin, microbes encounter dendritic cells.
Both the epidermis and dermis contain large numbers of antigen-
trapping dendritic cells. The best known of these are Langerhans

cells. Langerhans cells bind exogenous antigen and present it to
nearby helper T cells. They account for 50% to 70% of the dendritic
cells in pig epithelium. The dermis also contains resident dendritic
cells, as does the subcutaneous adipose tissue layer.

Healthy skin resident T cells are mainly located in the basal layer
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associated with Langerhans cells. CD4 and CD8 cells are present
in equal numbers. In humans and mice they are predominantly α/β
T cells. In some domestic mammals, many are γ/δ T cells. In cattle,

for example, 44% of dermal T cells are γ/δ positive. The three major
Th cell subsets are present. The microbiota regulate the
development of immunity in the skin and intestine. For example,
Staphylococcus epidermidis induces Th17 cells in the skin, and these

skin Th17 cells are regulated by the skin microbiota independently
of the intestinal microbiota. A subset of circulating T cells that home
to the skin and produce IL-22 (Th22 cells) have also been identified.
IL-22 plays an important role in maintaining barrier function on

exposed body surfaces. It promotes antimicrobial immunity,
inflammation, and tissue repair. Human skin also contains resident
memory T cells that can provide global skin immunity and thus
protect noninvolved skin. Treg cells facilitate cutaneous wound

healing by enhancing local production of the epithelial growth
factor receptor.
Ruminant γ/δ T cells recirculate continuously between epithelial
surfaces such as the skin or intestinal epithelium and the

bloodstream. In sheep, they are located in skin near the basal layer
of the epidermis and in the dermis close to hair follicles and
sebaceous glands. They are uncommon in wool-covered skin but
are present in large numbers in bare and hairy skin. They are also
found in the epithelium of the tongue, esophagus, trachea, and

bladder. γ/δ T cells are a major population in skin-draining afferent

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