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SECTION I
Cellular & Molecular Basis for Medical Physiology
GRANULOCYTES
All granulocytes have cytoplasmic granules that contain bio-
logically active substances involved in inflammatory and aller-
gic reactions.
The average half-life of a neutrophil in the circulation is 6
hours. To maintain the normal circulating blood level, it is
therefore necessary to produce over 100 billion neutrophils per
day. Many neutrophils enter the tissues, particularly if trig-
gered to do so by an infection or by inflammatory cytokines.
They are attracted to the endothelial surface by cell adhesion
molecules known as selectins, and they roll along it. They then
bind firmly to neutrophil adhesion molecules of the integrin
family. They next insinuate themselves through the walls of the
capillaries between endothelial cells by a process called
dia-
pedesis.
Many of those that leave the circulation enter the gas-
trointestinal tract and are eventually lost from the body.
Invasion of the body by bacteria triggers the
inflammatory
response.
The bone marrow is stimulated to produce and
release large numbers of neutrophils. Bacterial products inter-
act with plasma factors and cells to produce agents that attract
neutrophils to the infected area
(chemotaxis).
The chemotac-
tic agents, which are part of a large and expanding family of
chemokines
(see following text), include a component of the
complement system (C5a); leukotrienes; and polypeptides
from lymphocytes, mast cells, and basophils. Other plasma
factors act on the bacteria to make them “tasty” to the phago-
cytes
(opsonization).
The principal opsonins that coat the
bacteria are immunoglobulins of a particular class (IgG) and
complement proteins (see following text). The coated bacteria
then bind to receptors on the neutrophil cell membrane. This
triggers, via heterotrimeric G protein-mediated responses,
increased motor activity of the cell, exocytosis, and the so-
called respiratory burst. The increased motor activity leads to
prompt ingestion of the bacteria by endocytosis
(phagocyto-
sis).
By
exocytosis,
neutrophil granules discharge their con-
tents into the phagocytic vacuoles containing the bacteria and
also into the interstitial space
(degranulation).
The granules
contain various proteases plus antimicrobial proteins called
defensins.
In addition, the cell membrane-bound enzyme
NADPH oxidase
is activated, with the production of toxic
oxygen metabolites. The combination of the toxic oxygen
metabolites and the proteolytic enzymes from the granules
makes the neutrophil a very effective killing machine.
Activation of NADPH oxidase is associated with a sharp
increase in O
2
uptake and metabolism in the neutrophil (the
respiratory burst
) and generation of O
2
- by the following
reaction:
NADPH + H
+
+ 2O
2
+
→
NADP
+
+ 2H
+
+ 2O
2O
2- is a
free radical
formed by the addition of one electron
to O
2
. Two O
2
react with two H
- to form H
2
O
2
in a reaction
catalyzed by the cytoplasmic form of superoxide dismutase
(SOD-1):
O
2
- O
2
- H
- H
- SOD-1
→
H
2
O
2- O
2
- O
- SOD-1
- H
- H
- O
O
2- and H
2
O
2
are both oxidants that are effective bacteri-
cidal agents, but H
2
O
2
is converted to H
2
O and O
2
by the
enzyme
catalase.
The cytoplasmic form of SOD contains both
Zn and Cu. It is found in many parts of the body. It is defec-
tive as a result of genetic mutation in a familial form of
amyo-
trophic lateral sclerosis
(ALS; see Chapter 19). Therefore, it
may be that O
2
accumulates in motor neurons and kills them
in at least one form of this progressive, fatal disease. Two
other forms of SOD encoded by at least one different gene are
also found in humans.
Neutrophils also discharge the enzyme
myeloperoxidase,
which catalyzes the conversion of Cl
, Br
, I
, and SCN
to
the corresponding acids (HOCl, HOBr, etc). These acids are
also potent oxidants. Because Cl
is present in greatest abun-
dance in body fluids, the principal product is HOCl.
In addition to myeloperoxidase and defensins, neutrophil
granules contain an elastase, two metalloproteinases that attack
collagen, and a variety of other proteases that help destroy
invading organisms. These enzymes act in a cooperative fash-
ion with the O
2
, H
2
O
2
, and HOCl formed by the action of the
NADPH oxidase and myeloperoxidase to produce a killing
zone around the activated neutrophil. This zone is effective in
killing invading organisms, but in certain diseases (eg, rheuma-
toid arthritis) the neutrophils may also cause local destruction
of host tissue.
The movements of the cell in phagocytosis, as well as
migration to the site of infection, involve microtubules and
microfilaments (see Chapter 1). Proper function of the
microfilaments involves the interaction of the actin they con-
tain with myosin-1 on the inside of the cell membrane (see
Chapter 1).
Like neutrophils,
eosinophils
have a short half-life in the
circulation, are attracted to the surface of endothelial cells by
selectins, bind to integrins that attach them to the vessel wall,
and enter the tissues by diapedesis. Like neutrophils, they
release proteins, cytokines, and chemokines that produce
inflammation but are capable of killing invading organisms.
However, eosinophils have some selectivity in the way in
which they respond and in the killing molecules they secrete.
Their maturation and activation in tissues is particularly stim-
ulated by IL-3, IL-5, and GM-CSF (see below). They are espe-
cially abundant in the mucosa of the gastrointestinal tract,
where they defend against parasites, and in the mucosa of the
respiratory and urinary tracts. Circulating eosinophils are
increased in allergic diseases such as asthma and in various
other respiratory and gastrointestinal diseases.
Basophils
also enter tissues and release proteins and cyto-
kines. They resemble but are not identical to mast cells, and
like mast cells they contain histamine (see below). They
release histamine and other inflammatory mediators when
activated by binding of specific antigens to cell-fixed IgE mol-
ecules, and are essential for immediate-type hypersensitivity
reactions. These range from mild urticaria and rhinitis to
severe anaphylactic shock. The antigens that trigger IgE for-
mation and basophil (and mast cell) activation are innocuous
to most individuals, and are referred to as allergens.