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The body defenses
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The body's defenses
Nonspecific mechanisms provide general barriers to infection:
The microbe must penetrate an external barrier formed by the skin and
mucous membrane, which cover the surface and line the openings of an
animal's body.
If it succeeds, the pathogen encounters the second line of nonspecific
defense:
Nonspecific defense mechanisms
Specific mechanisms
(Immune system)
First line of defense
-Skin
Second line of defense
Third line of defense
-Phagocytic white blood -Lymphocytes
cells
-Mucous
membranes -Antimicrobial proteins
-Antibodies
and their secretions
-The
inflammatory
response
-The skin and mucous membranes:
The intact skin is a barrier that can't normally penetrated by bacteria or
viruses. The mucous membranes that lining the digestive, respiratory,
genitourinary tracts bar the entry of potentially harmful microbes.Skin and
mucous membranes also constitutes a chemical barriers by secreting.
Example: Secretions from oil and sweat glands give the skin a pH ranging
from 3-5, which is acidic enough to discourage many microorganisms from
colonizing there.
Saliva, tears, and mucous secretions contain antimicrobial protective
protein called lysozyme. an enzyme that digests the cell walls of many
bacteria and destroys many microbes entering the upper respiratory system
and the openings around the eyes.
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Mucous:
The viscous fluid secreted by cells of the mucous membranes, also traps
particles that contact it. Microbes entering the upper respiratory system are
often caught in the mucus and are then swallowed or expelled.
Lining the trachea are specialized epithelial cells equipped with cilia that
sweep out microbes and other particles trapped by the mucus, preventing
them from entering the lungs.
Microbes present in food passes through the highly acidic gastric juice inside
the stomach which destroys most microbes before passing to the intestinal
tract.
Phagocytic white cells and natural killer cells:
The body's internal mechanisms of nonspecific defense depend mainly on
phagocytosis, the ingestion of invading particles by certain types of white
blood cells.
The phagocytic cells called neutrophils comprise about 60% to 70% of all
WBCs. Attracted by chemical signals, neutrophils can leave the blood and
enter infected tissue by amoeboid movement, destroying microbes there.
The migration toward the source of a chemical attractant is called
chemotaxis.
Also, neutrophils tend to self-destruct and their average life is only a few days.
Monocytes, performs 5% of WBCs, provide an even more effective
phagocytic defense. After maturation, monocytes circulate in blood for hours,
then migrate into tissues, enlarging and developing into macrophages (big
eaters).
Macrophages, the largest phagocytic cells, are especially effective, long-living
phagocytes. These amoeboid cells extend pseudopodia that pull in microbes.
(Fig. 39.3)P854.and destroys it by digestive enzymes and reactive forms of
oxygen within the macrophages.
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Mechanisms for evading phagocytosis:
1. Certain bacteria, have special capsules to which the macrophage can't
attach.
2. Others have developed a resistance to the lytic enzymes of the phagocyte
and can even reproduce within macrophages.
Some macrophages reside permanently in organs and connective tissues.
In the lungs, for example, they are alveolar macrophages; in the liver, they
are called Kupffer's cells.
Fixed macrophages are especially numerous in the lymph nodes and spleen,
key organs of the lymphatic system (Fig. 39.4) mnp855.
Eosinophils, about 1.54% of WBCs which have only limited phagocytic
activity but contain destructive enzymes within cytoplasmic granules.
It function against larger parasitic invaders, such as worms. Eosinjophils
position themselves against the external wall of a worm and discharge the
destructive enzymes from their granules.
Natural killer cells:
They do not attack microorganisms directly, but rather destroy the body's own
infected cells, especially cells harboring viruses, which can reproduce only
within host cells. The natural killers also assault aberrant cells that could form
tumors, by attacking the membrane of the target cell, which causes that cell to
lyse (break open).
Antimicrobial proteins
A variety of proteins function in nonspecific defense either by attacking
microorganisms directly or by impeding their reproduction.
The complement system: is a group of at least 20 proteins, named for its
cooperation with (complementation) other defense mechanisms.
The complement proteins act together in a cascade of activation steps that
culminates with lysis of invading microbes.
Some components of the complement system also function in chemotaxis as
attractants for the recruitment of phagocytes to sites of infection.
The body defenses
(Fig. 39.3 P854)and (fig. 39.4 p855)
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Interferons:
First identified in 1957, where it is substances that virus-infected cells
produce, helping other cells interfere with, or resist infection by, the virus.
Types of interferon (alpha, beta, and gamma).
Interferons are secreted by an infected cell as an early, nonspecific defense
before specific antibodies appear.
Interferons are most effective in controlling short-term infections, such as cold
and influenza. In addition to its role as an antiviral agent, interferon-gamma
activates phagocytes, enhancing their ability to ingest and kill microorganisms.
The inflammatory response
Damage to tissue by a physical injury, such as a cut, or by the entry of
microorganisms, triggers an inflammatory response (Fig. 39.5) P 856
Vasodilation occurs, then blood supply increase to the infected area (redness
and heat). Blood vessles becomes more permeable and edema occurs. There
are chemical signals where histamine secreted by basophils and mast cells.
Histamine triggers local vasodilation. Also WBCs release prostaglandin.
Blood clotting is another sign of repair process. Phagocyte migration from
blood to the infected area is an important factor in immune response.
Neutrophils reach first then monocytes which change to macrophages.
The pus that often accumulates at the site of an infection consists of mostly of
dead cells and the fluid that leaked from the capillaries during the
inflammatory response. Another systemic response to infection is fever. Some
WBCs has the ability to secrete a molecules called pyrogens which can set
body thermostat at high temperature. A very high fever is dangerous but
moderate temperature may share in defense. Where it inhibit the growth of
some microorganisms by decreasing the amount of iron available.
The body defenses
Fig. 39.5 p856
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The immune system defends the body against specific invaders: an
overview
First line of defense
Second line of defense
Third line of defense
Microorganisms
The immune system develops a specific response against each type of foreign
microbe, toxin, or transplanted tissue.
Key features for the immune system
1. Specificty: The immune system has the ability to recognize and eliminate
particular microorganism and foreign molecules, a certain strain of flu virus, for
example. A foreign substance that elicits this immune response is called an
antigen.
The immune system respond to an antigen by activating specific proteins
called antibodies. Antigens that rigger an immune response includes
molecules belonging to viruses, bacteria, fungi, protozoa, and parasitic worms.
Antigenic molecules also mark the surfaces of such foreign materials as
pollen, insect venom, and transplanted tissue, such as skin or organs.
2. Diversity: The immune response has the ability to respond to millions of
kinds of invaders, each recognized by its antigenic markers. This diversity of
response is possible because there variety of lymphocytes populations, each
population bearing receptors for a particular antigen.
3. Memory: The immune system has the ability to "remember" antigens it has
encountered and to react to them more promptly and effectively on
subsequent exposures.
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This is called acquired immunity. Example: If we had chickenpox as a child,
we are unlikely to get it again.
4. Self/Nonself recognition: The immune system distinguishes the body's
own molecules from foreign molecules (antigens).Failure of self/nonself
recognition can lead to autoimmune disorders, in which the immune system
destroys the body's own tissues.
Active versus passive acquired immunity:
Immunity donated by recovering from an infectious disease such as
chickenpox is called active immunity because it depends on the response of
person's own immune system.
Active immunity
Naturally acquired
Response of person's own immune
system example: chickenpox
Artificially acquired
Response of person's own immune
system example: vaccination
(vaccine is may be inactivated
bacterial toxins, killed
microorganisms, or living but
weakened microorganisms.
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Antibodies can also be transferred from one individual to another, providing
passive immunity. (passive immunity persists for only few weeks or months).
This occurs naturally when preganant women's
body passes some of her antibodies across the
placenta to the fetus.
Certain antibodies are also
passed from the mother to
her nursing infant in breast
milk (colostrum).
Humoral immunity and cell-mediated immunity:
The immune system can actually mount two different types of responses to
antigens:
Humoral response
Results in the production of antibodies, which
are secreted by certain lymphocytes. Then
circulates in blood as soluble proteins in blood
plasma and lymph.
The circulating antibodies of the humoral
branch defend mainly against toxins, free
bacteria, and viruses present in body fluids.
Cell-mediated response
Depends on the direct
action of cells (certain types
of lymphocytes, which are
active against fungi,
protozoa, and worms.
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Cells of the immune system:
The vertebrate body is populated by two main classes of lymphocytes:
B-cells (B lymphocytes): which
carry out the humoral immune
response
T-cells (T lymphocytes), which
function mainly in the cell-mediated
immune response.
Lymphocytes originate from pluripotent stem cells in bone marrow, or in the
fetus, mainly in liver. Lymphocytes are alike; they differentiate into T cells or B
cells, depending on where they continue their maturation (Fig. 39.6) p858
Lymphocytes that migrate from the bone marrow to the thymus, a gland in the
upper region of the chest, develop into T cells (T for thymus). Lymphocytes
that remain in the bone marrow and continue their maturation there become B
cells. Mature B cells and T cells are most concentrated in lymph nodes, the
spleen. Both B cells and T cells are equipped with specific antigen receptors
on their plasma membranes. T cells receptors recognize antigens as
specifically as antibodies. When antigens bind to specific receptors on the
surface of a lymphocyte, the lymphocyte is activated to divide and
differentiate, giving rise to a population of effector cells, the cells that actually
defend the body in an immune response.
In the case of humoral response, B cells activated by antigen binding give rise
to effector cells called plasma cells, which secrete antibodies that help
eliminate that particular antigen.
Cytotoxic T cells (Tc) kill infected cells and cancer cells. Helper T cells (TH)
secrete protein factors called cytokines, which regulate both B cells and T
cells so it play an important role in humoral and cell-mediate responses.
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Clonal selection of lymphocytes is the cellular basis for ummunological
specificity and diversity:
How immune system response to millions of potential antigens? Fig. 39.7.
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The molecular basis of antigen-antibody specificity:
Most antigens are proteins or large polysaccharides. The molecules are often
outer components of the cell walls of bacteria, capsules, coats of viruses.
Transplanted organs, blood cells from individuals and species incite an
immune response. The surfaces of foreign substances such as pollen also
include antigens.
Antibodies do not generally recognize the antigen as a whole molecule, but it
recognized a specific site on the antigen surface called antigenic
determinant or epitope. Fig. 39.10 p 862
The body defenses
Antibodies constitute a class of proteins called immunoglobulins (Igs).
Every antibody molecule has at least two identical sites that bind to the
epitope that provoked its production. Five types of Ig are known:
IgG, IgM, IgA, IgD, and IgE.
Fig. 39.11 P 863
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The body defenses
How antibodies work:
Fig. 39.12 P865
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