Page 333 - Veterinary Immunology, 10th Edition

Graft rejection Suppressed No effect
VetBooks.ir Plasma cells in lymphoid tissues Minor drop Disappear
Presence of lymphocytes in T-independent areas Minor depletion Disappear
Serum immunoglobulins
Minor drop
Major drop
Antibody formation
Major drop
Minor effects
The results of thymectomy indicate that the neonatal thymus is
the source of most blood lymphocytes and that these lymphocytes
are mainly responsible for mounting cell-mediated immune
responses. They are called thymus-derived lymphocytes or T cells.
T-cell precursors originate in the bone marrow but then enter the
thymus. Once within the thymus, the cells (called thymocytes)
divide rapidly. Of the new cells produced, most die by apoptosis,

whereas the survivors (about 5% of the total in rodents and about
25% in calves) remain in the thymus for 4 to 5 days before leaving
and colonizing the secondary lymphoid organs.

T cells that enter the thymus have two conflicting tasks. They
must recognize foreign antigens, but, at the same time, must not
respond strongly to normal body constituents (self-antigens). A
two-stage selection process in the thymic medulla accomplishes this
feat. Thus thymocytes with receptors that bind self-antigens

strongly and that could therefore cause autoimmunity are killed by
apoptosis. Thymocytes with receptors that cannot bind any major
histocompatibility complex (MHC) class II molecules and thus

cannot react to any processed antigen are also killed.
On the other hand, those thymocytes that survive this “negative
selection” process but can still recognize specific MHC class II-
antigen complexes with moderate affinity are stimulated to grow—
a process called positive selection. These surviving cells eventually

leave the thymus as mature T cells, circulate in the bloodstream,
and colonize the secondary lymphoid organs.
Thymic epithelial cells are unusual since they express more than

400 antigens normally expressed in other tissues. In addition, these
cells have a very high level of autophagy. As a result, their
intracellular antigens are bound to MHC class II molecules and
expressed in large amounts on the epithelial cell surfaces. This
“promiscuous” antigen presentation ensures that developing

thymocytes are presented with an unusually diverse array of
normal tissue antigens. Since T cells with receptors that bind and
respond to these antigens will die, the system ensures that those T

cells leaving the thymus lack receptors for most self-antigens and,

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