Page 337 - Veterinary Immunology, 10th Edition

administration of testosterone. Bursectomized birds have very low
VetBooks.ir levels of antibodies in their blood, and antibody-producing cells

disappear from lymphoid organs. However, they still possess
circulating T cells and can reject foreign skin grafts. Thus,

bursectomy has little effect on the cell-mediated immune response.
Bursectomized birds are more susceptible than normal to
leptospirosis and salmonellosis but not to intracellular bacteria such
as Mycobacterium avium.

Thus the bursa is a primary lymphoid organ that functions as a
maturation and differentiation site for the cells of the antibody-
forming system. Lymphocytes originating in the bursa are therefore
called B cells. The bursa acts like the thymus insofar as immature

cells produced in the bone marrow migrate to the bursa. These cells
then proliferate rapidly, but 90% to 95% of these eventually die by
apoptosis; the negative selection of self-reactive B cells. Once their
maturation is completed, the surviving B cells emigrate to

secondary lymphoid organs.
Close examination shows that the bursa is not a pure primary
lymphoid organ because it can also trap antigens and undertake
some antibody synthesis. It also contains a small focus of T cells just

above the bursal duct opening. Several different hormones have
been extracted from the bursa. The most important of these is a
tripeptide (Lys-His-glycylamide) called bursin that activates B cells
but not T cells.

Peyer's Patches

Structure

Peyer's patches (PPs) are lymphoid organs located in the walls of
the small intestine. Their structure and functions vary among

species. Thus in ruminants, pigs, horses, dogs, and humans (group
I), 80% to 90% of the PPs are found in the ileum, where they form a
single continuous structure that extends forward from the ileocecal
junction. In young ruminants and pigs, the ileal PPs may be as long
as 2 m. Ileal PPs consist of densely packed lymphoid follicles, each

separated by a connective tissue sheath, and contain only B cells
(Fig. 12.6).

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