Page 696 - Veterinary Immunology, 10th Edition

species can remove more bacteria from the blood than can the liver
VetBooks.ir and spleen. There is debate as to whether lung macrophages are

effective antigen-presenting cells. A dense network of dendritic
cells is found within airway epithelium and alveoli.

Although it has long been believed that the respiratory tract is
sterile, it too has a normal bacterial microbiota. The composition of
this microbiota has a direct influence on the development of allergic
respiratory disease (Chapter 21). The lung is exposed to an array of

microorganisms with the very first breath an animal takes. Despite
this, it does not develop extensive inflammation. The early defense
of the lung is almost totally dependent on innate immune
pathways. The inhaled organisms trigger signaling cascades that

result in the generation of surfactants, defensins, interferons,
lactoferrin, and oxidants. These are critical protective pathways.

Immunity in the Urogenital Tract

In the urinary system, the flushing action and low pH of urine
generally provide adequate protection; however, when urinary
stasis occurs, urethritis resulting from the unhindered ascent of
pathogenic bacteria is not uncommon.
The female reproductive tract can be divided into a lower part

(vagina and cervix) and an upper part (uterus and fallopian tubes).
The lower part is covered by stratified squamous epithelium, the
upper by columnar epithelium. All are covered by microbiocidal

mucus. Within the lower part, the keratinocytes express PRRs and
produce cytokines and antimicrobial peptides. The upper part
contains large numbers of macrophages, dendritic cells, and innate
lymphoid cells. The predominant immunoglobulin in
cervicovaginal mucus is IgA, whereas within the uterus, it is IgG. If

bacteria such as Campylobacter fetus infect the genital tract, vaginal
IgA antibodies immobilize and agglutinate the organisms. If the
mucous membrane becomes inflamed, IgG antibodies from serum

will also assist in protection. Surfactant protein A is also important
in protecting the vagina from infection. C. fetus infections are
associated with the presence of many mononuclear cells as well
with delayed skin reactions (type IV hypersensitivity) so that cell-
mediated immunity is also involved in resistance to this local

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