Page 43 - Veterinary Immunology, 10th Edition

to recognize selected foreigners or dissidents, keep a file on them,
VetBooks.ir and respond more promptly when they reappear.

It must be emphasized, however, that just as human societies and
responses are very complex and involve the interactions of

thousands of individuals, so too is the immune system. While, for
reasons of simplicity and teaching, we discuss discrete cells,
processes and pathways, the immune systems should be thought of
as an interactive network. Thousands of different cells interact in

many ways and are subject to multiple influences. The cells
involved interact with each other, sometimes in a very complex
manner. Likewise, the invading microbe, its virulence, its ability to
evade defenses, and its interactions with other microbes, all lead to

variations in a host's immune response.
For introductory purposes, we can consider the process of
adaptive immunity to proceed by a series of steps (Fig. 1.9). Thus it
is triggered by cells that can recognize, trap, and process antigen.

The most important of these cells are dendritic cells and
macrophages. These cells then present the antigen to the T and B
cells of the immune system. The T and B cells can recognize and
respond to this processed antigen since they possess specific

antigen receptors on their surface. The B cells, once activated, will
produce specific antibodies, while the T cells will participate in the
cell-mediated immune responses. Long-lived B and T memory cells
are generated at the same time. These cells retain the memory of

these events and will react very rapidly to each specific antigen if it
is encountered again. They are thus responsible for the enhanced
immunity that develops in secondary immune responses. Finally,
helper and regulatory T cells control these responses and ensure

that they function at an appropriate level.

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