Page 621 - Veterinary Immunology, 10th Edition

VetBooks.ir Antigen Regulation of Immune

Responses

Adaptive immune responses are antigen driven. They commence
only on exposure to an antigen, and when it goes, they stop. If an
antigen persists, the stimulus persists and the immune response is
prolonged. Prolonged responses occur after immunization with

slowly degraded antigens, such as the bacterial polysaccharides, or
with antigens incorporated in oil or insoluble adjuvants. Antigens
that do not reach organized lymphoid tissues, irrespective of their
origin, may fail to induce either immunity or tolerance. Thus self-

antigens restricted to sites such as the brain, or viruses such as
papillomaviruses that never enter lymphoid organs, are usually
ignored by the immune system.
Antibody responses are also regulated by antigen. Rigid

polymeric antigens such as those on a bacterial surface or antigens
linked to TCR activators such as LPS can induce B cell responses in
the absence of T cell help. On the other hand, non-polymeric,
flexible antigens such as soluble proteins induce B cell responses
+
only in the presence of CD4 T cells.

Antigen Processing and Immune Regulation

The nature of the immune response may vary in different parts of

the body depending on dendritic cell populations. Langerhans cells
in the skin seem especially suited for promoting T cell responses,
whereas follicular dendritic cells prime B cells. cDC1 cells are
optimized to present antigens to Th1 cells, whereas cDC2 cells
present antigens to Th2 cells. Adjuvants also influence the type of

immune response through their effects on APCs (Chapter 24).

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