Page 942 - Veterinary Immunology, 10th Edition

development of resistance.
VetBooks.ir Cell-Mediated Immunity

Live cysts of the tapeworm Taenia solium trigger a Th2 response and
thus IgE production. However, after the cysts die, they stimulate a

Th1 response and granuloma formation. Biopsies show IL-12, IL-2,
and IFN-γ associated with the granulomas surrounding dying
tapeworm cysts. It may be that the Th1 response occurs only when
the parasite can no longer influence the host's immune response.
Sensitized T cells attack helminths by two mechanisms. First,

mononuclear cells are attracted to the site of larval invasion and
render the local environment unsuitable for growth or migration.
Second, cytotoxic lymphocytes may kill the larvae. Thus treatment

of experimental animals with the T cell stimulant, bacille Calmette-
Guérin (BCG) vaccine (Chapter 41), inhibits the metastases of
hydatid cysts (Echinococcus granulosus). In tapeworm infestations in
which the parasite cyst (metacestode) grows within the host, the
parasite must obtain protein for nourishment. However, the

cysticerci of Taenia ovis actually grow larger in the presence of
immune serum than nonimmune serum. The parasites possess Fc
receptors, and it is possible that host immunoglobulins may feed

the parasite. Since cyst fluid contains lymphocyte mitogens, it has
also been suggested that these might stimulate productions of
immunoglobulins that may then be used by the parasite.
The complexity of resistance to helminths is well demonstrated in
sheep bred for resistance to Haemonchus contortus. Compared with

susceptible sheep, there are differences in B cell function; resistant
sheep have significantly more IgA- and IgG1-containing cells. There
is also evidence for differences in T cell function because resistant

sheep respond better to a T-dependent antigens, and treatment of
resistant lambs with a monoclonal antibody to CD4 completely
blocks their resistance to H. contortus. Mucosal mast cell numbers
and tissue eosinophilia are also reduced in these treated sheep. In
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contrast, depletion of CD8 cells has no effect on resistance.

Evasion of the Immune Response

Although there are multiple mechanisms whereby animals resist

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