Page 693 - Veterinary Immunology, 10th Edition

cells, whereas SP-D enhances their uptake and presentation of
VetBooks.ir antigen. Both SP-A and SP-D inhibit T cell proliferation.

The mucus layer is in continuous flow, being carried from the
bronchioles up the bronchi and trachea by ciliary action or

backward through the nasal cavity to the pharynx. Here the dirty
mucus is swallowed and digested in the intestinal tract. Particles
that bypass this mucociliary escalator and reach the alveoli are
phagocytosed by alveolar macrophages. Once these cells have

successfully ingested particles, they migrate to the mucus escalator
and are also carried to the pharynx.
The respiratory tract contains bronchial lymphoid nodules as
well as lymphocytes distributed diffusely throughout the lung and

airways (Fig. 22.3). The mucosa of the larynx contains many
immunologically active cells, including large numbers of T cells.
Antigen-sampling M cells may be associated with these lymphoid
nodules as well as with nasal mucosal lymphoid tissues. These

tissues mainly produce secretory IgA, especially in the upper
regions of the respiratory tract. This IgA binds to mucus through
secretory component and so enhances the clearance of adherent
bacteria. The polymeric immunoglobulin receptor (pIgR) is

expressed at low levels on bronchial epithelial cells. In the
bronchioles and alveoli, however, the secretions contain a large
proportion of IgG, the concentration of which is intermediate
between the levels in the trachea and in serum. IgE is also

synthesized in significant amounts in the lymphoid tissues of the
upper respiratory tract. As on other body surfaces, IgA in the
respiratory tract probably protects by immune exclusion, whereas
IgG and IgE act by immune elimination (Fig. 22.4).

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