Page 692 - Veterinary Immunology, 10th Edition

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FIG. 22.2 Some of the innate mechanisms involved in the
protection of the respiratory tract against infection and the influence
of particle size on the site of deposition of particles within the
respiratory tract. Note that only the smallest particles can penetrate
deeply and gain access to the alveoli.

A blanket of mucus gel produced by goblet cells lines the upper

respiratory tract. The mucus contains soluble host defense
molecules such as lysozyme, lactoferrin, surfactant proteins, and
cationic peptides such as the defensins and cathelicidins. Most
microorganisms that enter the mucus layer are likely to be killed

rapidly. There are four major surfactant proteins in lung fluid (SP-
A, -B, -C, -D) produced by alveolar type II cells. SP-B and -C are
extremely hydrophobic. Their function is to reduce surface tension
at the alveolar surface so that a thin film of fluid forms on the

surface and prevents lung collapse. SP-A and -D, in contrast, are C-
type lectins that bind to microbial surface carbohydrates and act as
opsonins. SP-A and -D also activate macrophages, promote
chemotaxis, enhance the respiratory burst, and the production of

inflammatory cytokines. These surfactant proteins also enhance the
clearance of apoptotic cells from the lung. This is especially
important in the resolution of inflammation, since apoptotic
neutrophils should be removed by macrophages as promptly as

possible. Surfactant proteins can also modulate the functions of
dendritic cells and T cells. SP-A inhibits the maturation of dendritic

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