628
UNIT FOUR
can elicit an immune response are called
antigens
(an
tı˘-
jenz). Lymphocytes and macrophages that recognize specif
c
nonselF antigens carry out adaptive immune responses.
Antigens
BeFore birth, cells inventory the proteins and other large
molecules in the body, learning to identiFy these as selF. The
lymphatic system responds to nonselF, or Foreign, antigens,
but not normally to selF antigens. Receptors on lymphocyte
surFaces enable these cells to recognize Foreign antigens.
Antigens may be proteins, polysaccharides, glycopro-
teins, or glycolipids. The antigens most eFFective in eliciting
an immune response are large and complex, with Few repeat-
ing parts. Sometimes, a smaller molecule that cannot by itselF
stimulate an immune response combines with a larger one,
which makes it able to do so (antigenic). Such a small mol-
ecule is called a
hapten
(hap
ten). Stimulated lymphocytes
react either to the hapten or to the larger molecule oF the
combination. Hapten molecules are in drugs, such as penicil-
lin; in household and industrial chemicals; in dust particles;
and in products oF animal skins (dander).
Lymphocyte Origins
During Fetal development, red bone marrow releases unspe-
cialized precursors oF lymphocytes into the circulation. About
halF oF these cells reach the thymus, where they remain For
a time. Here, these thymocytes specialize into T cells. (“T”
reFers to
thymus-derived
lymphocytes.) Later, some oF these
T cells constitute 70% to 80% oF the circulating lymphocytes
in blood
(f
g. 16.15)
. Other T cells reside in lymphatic organs
and are particularly abundant in the lymph nodes, the tho-
racic duct, and the white pulp oF the spleen.
Other lymphocytes remain in the red bone marrow
until they diFFerentiate Fully into
B lymphocytes,
or
B cells.
(Historically, the “B” stands For
bursa of Fabricius,
an organ
in the chicken where these cells were discovered.) The blood
distributes B cells, which constitute 20% to 30% oF circu-
lating lymphocytes. B cells settle in lymphatic organs along
Fever
A
fever
is a nonspecif c deFense that oFFers powerFul protec-
tion. A Fever begins as a viral or bacterial inFection stimulates
lymphocytes to proliFerate, producing cells that secrete a sub-
stance called
interleukin-1
(IL-1), more colorFully known as
endogenous pyrogen (“f
re maker From within”). IL-1 raises
the thermoregulatory set point in the brain’s hypothalamus
to maintain a higher body temperature.
±ever indirectly counters microbial growth because
higher body temperature causes the liver and spleen to
sequester iron, which reduces the level oF iron in the blood.
Bacteria and Fungi require iron For normal metabolism, so
their growth and reproduction in a Fever-ridden body slows
and may cease. Also, phagocytic cells attack more vigorously
when the temperature rises. ±or these reasons, low-grade
Fever oF short duration may be a desired natural response,
not a symptom to be treated aggressively with medications.
Table 16.3
summarizes the types oF innate (nonspeciFic)
deFenses.
RECONNECT
To Chapter 6, Clinical Application 6.4, page 184.
PRACTICE
15
What may cause an infection?
16
Explain seven innate (nonspeciF
c) defense mechanisms.
16.9
ADAPTIVE (SPECIFIC)
DEFENSES OR IMMUNITY
The
third line of defense,
immunity
(ı˘-mu
nı˘-te), is resis-
tance to specif
c pathogens or to their toxins or metabolic
by-products. An immune response is based upon the ability
to distinguish molecules that are part oF the body (“selF”) From
those that are not (“nonselF,” or Foreign). Such molecules that
TABLE
16.3
|
Types of Innate (NonspeciF
c) Defenses
Type
Description
Species resistance
A species is resistant to certain diseases to which other species are susceptible.
Mechanical barriers
Unbroken skin and mucous membranes prevent the entrance of some infectious agents. ±luids wash away microorganisms before they
can F
rmly attach to tissues.
Chemical barriers
Enzymes in various body ²
uids kill pathogens. pH extremes and high salt concentration also harm pathogens. Interferons induce
production of other proteins that block reproduction of viruses, stimulate phagocytosis, and enhance the activity of cells such that
they resist infection and the growth of tumors. Defensins damage bacterial cell walls and membranes. Collectins grab onto microbes.
Complement stimulates in²
ammation, attracts phagocytes, and enhances phagocytosis.
Natural killer cells
Distinct type of lymphocyte that secretes perforins that lyse virus-infected cells and cancer cells.
In²
ammation
A tissue response to injury that helps prevent the spread of infectious agents into nearby tissues.
Phagocytosis
Neutrophils, monocytes, and macrophages engulf and destroy foreign particles and cells.
±ever
Elevated body temperature inhibits microbial growth and increases phagocytic activity.
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