Page 318 - Anatomy and Physiology of Farm Animals, 8th Edition

Body Defenses and the Immune System / 303

agent and development of a response Many of the systemic changes characteris-
tic of a diseased animal are produced by
directed at that specific agent.
VetBooks.ir are among the first cells to respond during cytokines leaving a site of inflammation
Local tissue phagocytes (macrophages)
and entering the general circulation.
a nonspecific response; they attempt to Specific chemokines also attract more
engulf and destroy any foreign microbe or macrophages to areas of inflammation.
substance. The primary mode of destruc- Since macrophages are derived from circu-
tion is by intracellular fusion of phagocytic lating monocytes, part of this effect
vesicles with enzyme‐containing lys- requires that monocytes migrate from ves-
osomes. An accumulation of neutrophils in sels and develop into tissue macrophages.
the area is another component of the rapid The monocyte response is slower than that
nonspecific response. Recall that neutro- of neutrophils, so an accumulation of
phils are attracted to an area of injury and inflammatory cells that is primarily
inflammation by chemotactic factors and neutrophils is characteristic of an acute
those chemotactic factors may include sub- inflammatory response. An accumulation
stances released from damaged cells and by of macrophages is more characteristic of a
the invading microbes. Chemotactic factors chronic inflammatory response.
are also released by tissue macrophages as Another characteristic of an acute
part of their response to foreign microbes. inflammatory response is an increase in
The release of chemotactic factors by local blood flow and in movement of fluid
responding macrophages is one example of and plasma protein into the interstitial
a general scheme by which nonspecific and space. Agents that affect blood vessels in
specific immune responses are regulated. the local area of inflammation bring
That is, many of the cells participating in about these changes. Locally produced
the response secrete chemical messengers agents include eicosanoids (see
that affect the function of other respond- Chapter 15) that are produced by a vari-
ing cells. In this example, macrophages ety of cell types.
already responding secrete chemical agents Plasma proteins delivered to a site of
that act as chemotactic factors to attract inflammation include complement (also
neutrophils. In this way, higher levels of known as complement proteins). These
macrophage activity in response to greater are a group of plasma proteins that are nor-
numbers of microbes can promote the mally inactive in circulation (similar to the
accumulation of greater numbers of clotting factors), but some can be activated
neutrophils. by the presence of certain polysaccharide
Cytokine is the general term applied to components of the outer coverings of bac-
all chemical messengers (primarily pro- teria. The activated complement proteins
teins) that regulate cells involved in any can activate other complement proteins
immune response. Responding cells pro- (similar to the clotting cascades). The acti-
duce many of the cytokines, but other cell vated complement proteins have a number
types may also secrete them. More than of effects that contribute to the local
100 compounds have been classified as nonspecific response. These include: (1)
cytokines, which provides some insight leukocyte chemotaxis; (2) direct attack on
into the complexity of these responses. bacteria by increasing the permeability of
The various cytokines that function as their cell walls; (3) stimulation of hista-
chemotactic factors are collectively termed mine release from mast cells; and (4)
chemokines. Interleukin‐1 is a specific opsonization. Opsonization is the facilita-
cytokine that also functions as a pyrogen tion of engulfment by phagocytes, and any
(an agent that produces fever). The pyro- agent that can perform this facilitation is
gen effect of interleukin‐1 is but one exam- termed an opsonin. The local release of
ple of a systemic effect of a cytokine that is histamine causes a further increase in
unrelated to cells of the immune system. blood flow and accumulation of interstitial