Chapter 13
Innate and Adaptive Immunity
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Neutrophils and Macrophages
The leukocytes involved in the innate immune response are
derived from myeloid stem cells and subdivided into two dis-
tinct groups based upon the presence or absence of specific
staining granules in their cytoplasm. Leukocytes that contain
granules are classified as granulocytes and include neutro-
phils, eosinophils, and basophils. Cells that lack granules are
classified as agranulocytes and include lymphocytes, mono-
cytes, and macrophages.
Neutrophils, which are named for their neutral-staining
granules, are the most abundant granulocytes found in the
body and make up approximately 55% of all white blood
cells. They are also known as polymorphonuclear neutrophils
(PMNs). They are phagocytic cells and are capable of ame-
boid-like movement. They function as early responder cells
in innate immunity. They are rare in the tissues and in body
cavities and lay predominantly dormant in the blood and
bone marrow until they are needed in the immune response.
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Eosinophils have large coarse granules and normally com-
prise only 1% to 4% of the total white cell count. In contrast to
neutrophils, these cells do not ingest cellular debris but rather
antigen–antibody complexes and viruses. They frequently
become active in parasitic infections and allergic responses.
Basophils make up less than 1% of the total white cell count
and contain granules that release a multitude of substances
including histamine and proteolytic enzymes. There function
is not completely understood, but they are believed to play a
role in allergy and parasitic infection as well.
The agranulocytes involved in innate immunity are part
of the mononuclear phagocyte system (MPS) and include the
monocytes and macrophages. Monocytes are the largest in
size of all the white blood cells but make up only 3% to 7%
of the total leukocyte count. They are released from the bone
saliva, and human milk, which is capable of cleaving the
walls of bacterial cells by hydrolyzing the 1,4 beta-linkages
between residues in peptidoglycan. The complement sys-
tem is found in the blood and is essential for the activity of
antibodies. It is comprised of 20 different proteins, many of
which act as precursors of enzymes. An antigen–antibody
complex initiates this system. Activation of the comple-
ment system increases bacteria aggregation, which renders
them more susceptible to phagocytosis through activation
of mast cells and basophils and through the direct release
of lytic complexes that rupture cell membranes of invad-
ing organisms (Fig. 13.1). In addition, recent research has
shown that complement plays a key role in bridging the
innate–adaptive immune responses through the release of
C3 and C5 from DCs.
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In the stomach and intestines, death
of microbes results from the action of digestive enzymes,
acidic conditions, and secretions of
defensins
, small cat-
ionic peptides that kill within minutes both gram-positive
and gram-negative microorganisms by disrupting the
microbial membrane.
When pathogens overcome the epithelial defenses, the
innate immune response is initiated by the body’s leukocytes
by the recognition of common surface receptors present on the
invading microorganisms.
Cells of Innate Immunity
The cells of the innate immune response are capable of recog-
nizing microbes that share common surface receptor charac-
teristics and in response initiate a broad spectrum of responses
that target the invading microorganisms. The key cells of
innate immunity include neutrophils, macrophages, DCs, NK
cells, and intraepithelial lymphocytes.
A
B
Leukocyte Lysosome
Vesicle
Bacterium
Lysosome
Digestive products
Residue
Blood
Capillary wall
Erythrocyte
Bacterium
Epithelial cell
FIGURE 13.1
•
Phagocytosis. (
A
) A phagocytic white blood cell moves through a capillary that is in
an infected area and engulfs the bacteria. (
B
) The lysosome digests the bacteria that was in a vesicle.
(From Cohen B. J. (2013).
Memmler’s the human body in health and disease
(12th ed.) Philadelphia, PA:
Lippincott Williams & Wilkins.)
