Download IMMUNITY Body Defenses Nonspecific Body Defenses [In native

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Medical Physiology
Z.H.Al-Zubaydi
IMMUNITY
Body Defenses
Our bodies normally are exposed to bacteria, viruses, fungi, and
parasites, which occur especially in the skin, the mouth, the respiratory
passageways, the intestinal tract, the lining membranes of the eyes, and
even the urinary tract. Many of these agents are capable of causing
serious disease if they invade the deeper tissues. In addition, we are
exposed intermittently to other highly infectious bacteria and viruses
besides those that are normally present in our bodies, and these cause
lethal diseases such as pneumonia, streptococcal infection, and typhoid
fever.
The body's defenders against these tiny organisms are two systems:
I.
Non Specific defense system responds immediately to protect the
body from all foreign substances.
II.
Specific defense system [ Immune System; a functional system
rather than an organ system] mount the attack against particular
foreign substances. The resulting highly specific resistance to
diseases is called immunity.
Nonspecific Body Defenses [In native Immunity]
the term nonspecific body defense refers to the mechanical barriers that
cover body surfaces and to cells and chemicals that act on the initial protection of the body from invading pathogens (harmful or disease-causing
microorganisms).
Surface Membrane Barriers
The body's first line of defense against the invasion of disease-causing
microorganisms is the skin and mucous membranes. As long as the skin is
unbroken, its keratinized epidermis is a strong physical barrier to most
microorganisms that swarm on the skin. Intact mucous membranes
provide similar mechanical barriers within the body, these membranes
produce a variety of protective chemicals:
1. The acid pH of skin secretions inhibits bacterial growth, and sebum
contains chemicals that are toxic to bacteria. Vaginal secretions of
adult females are also very acidic.
2. The stomach mucosa secretes hydrochloric acid and proteindigesting enzymes. Both kill pathogens.
3. Saliva and lacrimal fluid contain lysozyme, an enzyme that
destroys bacteria.
4. Sticky mucus traps many microorganisms that enter digestive and
respiratory passageways.
Some mucosae also have structural modifications that fend off potential
invaders [Mucus-coated hairs inside the nasal cavity trap inhaled
particles, and the ciliated mucosa of respiratory tract sweep dust].
Although the surface barriers are very effective, they are broken from
time to time by small nicks and cuts resulting [ex.; brushing teeth or
shaving]. When this happens and microorganisms do invade deeper
tissues, other nonspecific mechanisms come into play to defend the body.
Cells and Chemicals
For its second line of defense, the body uses an enormous number of cells
and chemicals to protect itself including:
1- Phagocytes
A phagocyte such as a macrophage or neutrophil, engulfs a foreign
particle much the way an amoeba ingests a food particle. Flowing
cytoplasmic extensions bind to the particle and then pull it inside,
enclosing it in a vacuole. The vacuole then fused with the enzymatic
contents of a lysosome. and its contents are broken down or digested.
2- Natural Killer Cells
Natural killer (NK) cells, which are a unique group lymphocytes that can
lyse and kill cancer cells and virus-infected body cells well before
immune system is enlisted in the fight. Unlike lymphocytes of the
immune system, which recognize and react only against specific virus
infected or tumor cells, natural killer cells are less picky. They attack the
target cell's membrane release a lytic chemical called perforins.
3- Inflammatory Response
The inflammatory response is a nonspecific response that is triggered
whenever body tissues are injured (in response to physical trauma, intense
heat, and irritating chemicals, as well as to infection by viruses and
bacteria). The four signs and major symptoms of an acute inflammation
are redness, heat, swelling, and pain.
The inflammatory process begins with a chemical "alarm." When cells
are injured, they release inflammatory chemicals, including histamine and
kinins, that (1) cause blood vessels in the involved area to dilate and
capillaries to become leaky, (2) activate pain receptors, and (3) attract
phagocytes and white blood cells to the area. (This latter phenomenon is
called chemotaxis because the cells are following a chemical gradient.)
4- Antimicrobial chemicals
The body's most important antimicrobial chemicals are:
I.
Complement proteins include 20 plasma proteins that circulate
in the blood in an inactivate state. When complement becomes
attached or fixed to foreign cells and form complement fixation
it is activated. One of this fixation results is the formation of
membrane attack complex (MAC) that produce lesion and holes
in the foreign cell's surface. These allow water to rush into the
cell causing it to brust.
II.
Interferon when viruses damage the body by entering tissue
cells to regenerate their ATP or to make proteins. The virus
infected cells secret small proteins called interferon that diffuse
to nearby cells and bind to their membrane receptors, this
binding hinders the ability of viruses to multiply within these
cells.
5- Fever although high fevers are dangerous because excess heat
scrambles enzymes and other body proteins, mild to moderate fever
seems to be benefit to the body. Bacteria require large amounts of iron
and zinc to multiply, but during a fever the liver and spleen gather up
these nutrients. Also fever increase the metabolic rate of tissue cells in
general, speeding up repair process
Specific Body Defenses [Acquired Immunity]
Sometimes referred to as the body's third line of defense, the
immune system is a functional system that recognizes foreign
molecules (antigens) and acts to inactivate or destroy them. Normally it
protects us from a wide variety of pathogens, as well as from abnormal
body cells.
There are three important aspects of the immune response:
1.
It is antigen specific—It recognizes and acts against particular
pathogens or foreign substances.
2.
It is systemic—Immunity is not restricted to the initial infection
site.
3.
It has "memory"—It recognizes and mounts even stronger
attacks on previously encountered pathogens.
Antigens
An antigen (Ag) is any substance capable of exciting our immune
system and provoking an immune response. Most antigens are large,
complex molecules that are not normally present in our bodies or
nonself.
It is also important to remember that our own cells are richly
studded with a variety of protein molecules (self-antigens).which they
are strongly antigenic to other people. This helps explain why our
bodies reject cells of transplanted organs or foreign grafts unless
special measures (drugs and others) are taken to cripple or stifle the
immune response.
Cells of the Immune System
The cells of the immune system are lymphocytes and macrophages.
Lymphocytes exist in two major types. The B lymphocytes, or B
cells, produce antibodies and oversee humoral immunity, whereas the
T lymphocytes, or T cells, are non-antibody producing lymphocytes
that constitute the cell-mediated arm of immunity. Unlike the two types
of lymphocytes, macrophages do not respond to specific antigens but
instead play an essential role in helping the lymphocytes.
Lymphocytes
Like all blood cells, lymphocytes originate from hemocytoblasts in
red bone marrow. The immature lymphocytes released from the
marrow are essentially identical. Whether a given lymphocyte
matures into a B cell or a T. T cells arise from lymphocytes that
migrate to the thymus, where they undergo a maturation process of 2
to 3 days, directed by thymic hormones (thymosin and others).
Within the thymus, the immature lymphocytes divide rapidly and their
numbers increase enormously, but only those maturing T cells have
the ability to identify foreign antigens. B cells develop
immunocompetence in bone marrow, but little is known about the
factors that regulate B cell maturation. After becoming
immunocompetent, both T cells and B cells migrate to the lymph nodes
and spleen.
Macrophages
Macrophages, which also become widely distributed throughout the
lymphoid organs and connective tissues, arise from monocytes formed
in the bone marrow. They act as antigen presenters in the specific
defense system. Macrophages also secrete cytokine proteins, called
monokines, that are important in the immune.
Humoral Immunity [Antibody-Mediated Response]
Immature B lymphocytes is stimulated to its development into a
fully mature B cells when an antigen binds to its surface receptors. This
binding event sensitizes, or activates, the lymphocyte to undergo clonal
selection. The lymphocyte begins to grow and then multiplies rapidly
to form cells all exactly like itself and bearing the same antigen-specific
receptors. The resulting is identical cells descended from the same
ancestor cell is called a clone, and clone formation is the primary
humoral response to that antigen. Most of the B cell clone members,
or descendants, become plasma cells. After an initial lag period, these
antibody-producing "factories" act in producing the same highly specific
antibodies at a rate of about 2000 antibody molecules per second. (The B
cells themselves produce only very small amounts of antibodies.)
However, this flurry of activity lasts only 4 or 5 days; then the plasma
cells begin to die. Antibody levels in the blood during this primary
response peak in about 10 days and then slowly decline.
B cell clone members that do not become plasma cells
become long-lived memory cells capable of responding to the
same antigen at later meetings with it. Memory cells are responsible
for the immunological "memory" mentioned earlier. These later
immune responses, called secondary humoral responses, are much
faster, more prolonged, and more effective because all the preparations for this attack have already been made (fig.13). Within hours
after recognition of the old antigen, a new army of plasma cells is being
generated, and antibodies begin to flood into the bloodstream. Within
2 to 3 days, blood antibody levels peak (at much higher levels than
seen in the primary response), and their levels remain high for weeks
to months.
Figure (13):Primary and secondary responses