Page 848 - Veterinary Immunology, 10th Edition

give good protection, that serum cannot confer protection, that
VetBooks.ir antibody levels do not relate to resistance, and that delayed

hypersensitivity reactions can be elicited to the bacterial antigens,
then type 1 responses probably play an important role in resistance,

and the use of vaccines containing living bacteria should be
contemplated.

Modification of Bacterial Disease by Immune

Responses

The immune response influences the course and severity of an

infection. At best, it will result in microbial destruction and a cure.
In the absence of a cure, however, the infection may be profoundly
modified. Much depends on whether a cell-mediated or antibody

response is generated. Thus the type of helper T cells induced
during infection affect the course of disease. As described in
Chapter 18, type 1 cell-mediated responses are required to control
intracellular bacteria since only activated macrophages can prevent
their growth. Macrophage activation requires that Th1 cells

produce IFN-γ. Once activated, the M1 cells may localize or cure
these infections. If an animal mounts an inappropriate Th2 response
so that M2 but not M1 macrophages are generated, chronic

progressive disease may result. This is seen in Johne's disease of
sheep. Some animals develop multibacillary (MB) disease, in which
their intestinal lesions contain enormous numbers of bacteria (Fig.
26.3) and little histological evidence of a cell-mediated response.
Their granulomas lack organization, with large numbers of

bacteria-laden macrophages intermixed with lymphocytes. In
contrast, other sheep may develop paucibacillary (PB) disease, in
which the lesions contain very few bacteria but large numbers of

lymphocytes. These are organized nodular lesions with epithelioid
cells and multinucleated giant cells at the center surrounded by
fibrous connective tissue. The two forms of the disease are
associated with differential expression of cytokine and chemokine
receptors. Thus animals with the PB disease have increased

+
numbers of CD25 T cells that produce more IL-2 and much more
IFN-γ than sheep with the MB form of the disease (Fig. 26.4). In
contrast, sheep with the MB disease have higher antibody levels

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