Defense Mechanisms of the Body
647
specifi c recognition, by lymphocytes, of the substance or cell to be attacked. This is followed by an attack
unique for that substance or cell. Nonspecifi c and specifi c immune responses function in synchrony. For
example, components of nonspecifi c immunity provide instructions that enable the specifi c responses to
select appropriate targets to attack and to have strategies for their elimination.
Before introducing the cells that participate in immune defenses, let us fi rst describe the microbes we will
be most concerned with in this chapter—bacteria and viruses. These are the dominant infectious organisms
in the United States and other industrialized nations. On a global basis, however, infections with parasitic
eukaryotic organisms are responsible for a huge amount of illness and death. For example, several hundred
million people now have malaria, a disease caused by infection of the
Plasmodium
species.
Bacteria
are unicellular organisms that have an outer coating (the cell wall) in addition to a plasma
membrane, but no intracellular membrane-bound organelles. Bacteria can damage tissues at the sites of
bacterial replication, or they can release toxins that enter the blood and disrupt physiological functions
in other parts of the body.
Viruses
are essentially nucleic acids surrounded by a protein coat. Unlike bacteria, viruses lack both the
enzyme machinery for metabolism and the ribosomes essential for protein synthesis. Thus, they cannot
multiply by themselves but must exist inside other cells and use the biochemical apparatus of those cells.
The viral nucleic acid directs the host cell to synthesize the proteins required for viral replication, with
the required nucleotides and energy sources also supplied by the host cell. The effect of viral habitation
and replication within a cell depends on the type of virus. After entering a cell, some viruses (the
common cold virus, for example) multiply rapidly, kill the cell, and then move on to other cells. Other
viruses, such as the one that causes genital herpes, can lie dormant in infected cells before suddenly
undergoing the rapid replication that causes cell damage. Finally, certain viruses can transform their host
cells into cancer cells.
Cells Mediating Immune Defenses
The cells that carry out immune defenses collectively make up
the
immune system,
but they do not constitute a “system”
in the sense of anatomically connected organs like the gas-
trointestinal and urinary systems. Instead, they are a diverse
collection of cells found both in the blood and lymph and in
tissues throughout the body. Because of the large number of
cells and the far larger number of chemical messengers that
participate in immune defenses, a miniglossary defi
ning the
cells and messengers discussed in this chapter is given toward
the end of this chapter in Table 18–12.
The most numerous of the immune system cells are the
various types of white blood cells collectively known as
leuko-
cytes.
These include the neutrophils, basophils, eosinophils,
monocytes, and lymphocytes. (The fi rst three types are also
grouped under the general term
polymorphonuclear granulo-
cyte
because their cell nuclei have unusual, irregular appear-
ances.) The anatomy, production, and blood concentrations
of these cells were described in Chapter 12 and should be
reviewed at this time. Unlike erythrocytes, leukocytes use the
blood mainly for transportation and leave the circulatory sys-
tem to enter the tissues where they function.
Plasma cells
are not really a distinct cell line but dif-
ferentiate from a particular set of lymphocytes (the B lympho-
cytes) during immune responses. Despite their name, plasma
cells are primarily found in the tissues in which they differ-
entiate from their parent lymphocytes. The major function of
plasma cells is to synthesize and secrete antibodies.
Macrophages
are found in virtually all organs and tis-
sues, their structures varying somewhat from location to loca-
tion. They are derived from monocytes that pass through the
walls of blood vessels to enter the tissues and transform into
macrophages. In keeping with one of their major functions,
the engulfi ng of particles and microbes by endocytosis, mac-
rophages are strategically placed where they will encounter
their targets. For example, they are found in large numbers
in the various epithelia in contact with the external environ-
ment, such as the skin and internal surfaces of respiratory and
digestive system tubes. In several organs, they line the vessels
through which blood or lymph fl ows.
There are several cell populations that are not mac-
rophages and are not descended from monocytes, but exert
various macrophage functions. These are collectively termed
macrophage-like
(or
dendritic
)
cells.
They are found scat-
tered in almost all tissues.
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