Page 334 - Veterinary Immunology, 10th Edition

as a result, cannot respond to normal body components.
VetBooks.ir Thymic Hormones

Within the thymus, cells are regulated by a complex mixture of
cytokines and small peptides collectively known as thymic

hormones. These include peptides variously called thymosins,
thymopoietins, thymic humoral factor, thymulin, and the
thymostimulins. Thymulin is especially interesting because it is a
zinc-containing peptide secreted by the thymic epithelial cells, and
it can partially restore T cell function in thymectomized animals.

Zinc is an essential mineral for the development of T cells.
Consequently, zinc-deficient animals have defective cell-mediated
immune responses (Chapter 40). Hassall's corpuscles play a

functional role in regulating thymic activity since they produce a
growth factor called thymic stromal lymphopoietin (TSLP, Chapter
21). TSLP activates thymic dendritic cells that can stimulate
regulatory T cells and so controls the positive selection process.

Bursa of Fabricius

The bursa of Fabricius is found only in birds. It is a round sac
located just above the cloaca (Fig. 12.4). Like the thymus, the bursa

reaches its greatest size in the chick about 1 to 2 weeks after
hatching and then shrinks as the bird ages. It is very difficult to
identify in older birds.

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