648
Chapter 18
Mast cells
are found throughout connective tissues,
particularly beneath the epithelial surfaces of the body. They
are derived from the differentiation of a unique set of bone
marrow cells that have entered the blood and then left the
blood vessels to enter connective tissue, where they differenti-
ate and undergo cell division. Thus, mature mast cells, unlike
basophils, with which they share many characteristics, are not
normally found in the blood. The most striking anatomical
feature of mast cells is their very large number of cytosolic ves-
icles, which secrete locally acting chemical messengers such as
histamine.
The sites of production and functions of all these cells
are briefl y listed in
Table 18–1
for reference and will be
described in subsequent sections. Suffi ce it for now to empha-
size two points.
Lymphocytes
serve as recognition cells in
specifi c immune defenses and are essential for all aspects of
these responses. Neutrophils, monocytes, macrophages, and
dendritic cells have a variety of activities, but particularly
important is their ability to secrete infl
ammatory mediators
and to function as
phagocytes.
A phagocyte denotes any cell
capable of
phagocytosis,
the form of endocytosis whereby
the phagocytic cell engulfs and usually destroys particulate
matter.
Cytokines
The cells of the immune system secrete a multitude (more than
100, to date) of protein messengers that regulate host cell divi-
sion (mitosis) and function in both nonspecifi c and specifi c
immune defenses. The collective term for these messengers,
each of which has its own unique name, is
cytokines.
They
are not produced by distinct specialized glands but rather by
a variety of individual cells. The great majority of their actions
occur at the site at which they are secreted, the cytokine acting
as an autocrine/paracrine agent. In some cases, however, the
cytokine circulates in the blood to exert hormonal effects on
distant organs and tissues involved in host defenses.
Cytokines link the components of the immune system
together. They are the chemical communication network that
allows different immune system cells to “talk” to one another.
This is called
cross-talk,
and it is essential for the precise tim-
ing of the functions of the immune system. Most cytokines
are secreted by more than one type of immune system cell
and by nonimmune cells as well (for example, by endothelial
cells and fi broblasts). This often produces cascades of cyto-
kine secretion, in which one cytokine stimulates the release
of another, and so on. Any given cytokine may exert actions
on an extremely broad range of target cells. For example, the
cytokine interleukin 2 infl uences the function of virtually
every cell of the immune system. There is great redundancy
in cytokine action; that is, different cytokines can have very
similar effects. Cytokines are also involved in many nonim-
munological processes, such as bone formation and uterine
function.
This chapter will be limited to a discussion of a few of
the important cytokines and their most important functions,
which are summarized in
Table 18–2
.
Nonspecifi c Immune Defenses
Nonspecifi c immune defenses protect against foreign cells
or matter without having to recognize their specifi c identi-
ties. These defenses recognize some
general
property mark-
ing the invader as foreign. The most common such identity
tags are often found in particular classes of carbohydrates or
lipids that are in microbial cell walls. Plasma-membrane recep-
tors on certain immune cells, as well as a variety of circulating
proteins (particularly a family called complement) can bind to
these carbohydrates and lipids at crucial steps in nonspecifi c
responses. This use of a system based on carbohydrate and
lipid for detecting the presence of foreign cells is a key fea-
ture that distinguishes nonspecifi c defenses from specifi c ones,
which recognize foreign cells mainly by specifi c proteins the
foreign cells produce.
The nonspecifi c immune defenses include defenses at the
body surfaces, the response to injury known as infl
ammation,
and a family of antiviral proteins called interferons.
Defenses at Body Surfaces
The body’s fi rst lines of defense against microbes are the bar-
riers offered by surfaces exposed to the external environment.
Very few microorganisms can penetrate the intact skin, and
the various skin glands, salivary glands, and the lacrymal (tear)
glands all secrete antimicrobial chemicals. These may include
antibodies, enzymes such as lysozyme, which destroy bacterial
cell walls, and an iron-binding protein called lactoferrin. This
protein prevents bacteria from obtaining the iron they need to
function properly.
The mucus secreted by the epithelial linings of the respi-
ratory and upper gastrointestinal tracts also contains anti-
microbial chemicals, but more important, mucus is sticky.
Particles that adhere to it are prevented from entering the
blood. They are either swept by ciliary action up into the
pharynx and then swallowed, as occurs in the upper respira-
tory tract, or are phagocytized by macrophages in the various
linings.
Other specialized surface defenses are the hairs at the
entrance to the nose, the cough and sneeze refl exes, and the
acid secretion of the stomach, which kills microbes.
Infl
ammation
Infl
ammation
is the local response to infection or injury. The
functions of infl ammation are to destroy or inactivate foreign
invaders and to set the stage for tissue repair. The key media-
tors are the cells that function as phagocytes. As noted earlier,
the most important phagocytes are neutrophils, macrophages,
and dendritic cells.
In this section, infl
ammation is described as it occurs in
the nonspecifi c defenses induced by the invasion of microbes.
Most of the same responses can be elicited by a variety
of other injuries—cold, heat, and trauma, for example.
Moreover, we will see later that infl
ammation accompanies
many
specifi
c
immune defenses in which the infl
ammation
becomes amplifi ed.
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