Page 704 - Veterinary Immunology, 10th Edition

VetBooks.ir Mucosal Lymphoid Tissues

Because of the importance of preventing invasion through the
mucosa, these surfaces contain large amounts of lymphoid tissue.

Mucosal lymphoid tissues fall into two groups: sites where antigens
are processed and immune responses are initiated (inductive sites),
and sites where antibodies and cell-mediated responses are
generated (effector sites).

Inductive Sites

The mucosa-associated lymphoid tissues (MALTs) possess the three
cell types required to initiate adaptive immune responses: T cells, B
cells, and dendritic cells. These tissues include lymphoid tissues in

the eyelids, nasal mucosa, tonsils, pharynx, tongue, and palate
(collectively called Waldeyer's ring); Peyer's patches; solitary
lymphoid nodules; the appendix in the intestine; and numerous
lymphoid nodules in the lung. These lymphoid tissues are known

by their acronyms. Thus GALT (gut-associated lymphoid tissue) is
the collective term for all the lymphoid nodules, Peyer's patches,
and individual lymphocytes found in the intestinal walls. Similarly,
BALT is the acronym used for the bronchus-associated lymphoid

tissue in the lungs. These organized lymphoid tissues, unlike lymph
nodes, do not react to foreign antigens delivered through afferent
lymph but rather sample them directly from the surface.
The tonsils are especially important in inducing immunity on

mucosal surfaces. Some organisms, however, can overcome the
defenses of the tonsils and use them as a portal of entry into the
body. For example, pathogens such as bovine herpesvirus-1,
Mannheimia hemolytica, S. suis, and Mycobacterium tuberculosis can

persist indefinitely within the tonsils.
Peyer's patches are the most significant of the mucosal lymphoid
tissues. A newborn calf normally has about 100 Peyer's patches, and
these may cover as much as half of the ileal surface. In ruminants

and pigs, there are two types of Peyer's patch that differ in location,
structure, and function. The ileocecal Peyer's patches may be
primary lymphoid organs in some species, whereas the jejunal

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