Lec. 5
Immune system
Immunity: is defined as the ability of human
body to resist against harmful chemicals such as
toxins that released by microorganism which
tend to damage the tissues & organs.
Lymphocytes are key constituents of the immune
system. The lymphatic system includes
lymphocytes, lymph nodes, tonsils, spleen,
thymus gland, lymph and lymphatic vessels.
There are two types of immune defense
1. Innate immunity.
2. Acquired (adaptive) immunity.
A-Innate immunity: in this type of immunity
the body is born with the ability to recognize
and destroy certain substances but cannot
distinguish among different kinds of bacteria
and remember previous encounters. Its
components are:
1.Mechanical mechanisms: a. skin b. mucous
2. Chemical mediators.
when the tissue injury occurs either by bacteria or
chemical there is a release of chemical mediators (also
called chemotaxins or chemotactic factor ) from
infected area which
attract leukocytes especially
neutrophils to the infected area.. The process of
attraction is called chemotaix.
1. Vasodilatation. 2. Increased vascular permeability
resulting in edema 3. “walling off’’ the infected area.
There are two types of inflammation: a. localized b.
Signs and symptoms of inflammation are: heat,
redness, pain, swelling, loss of function and release of
chemicals called pyrogens from leukocytes which cause
fever and inhibit growth of microorganism.
Pus formation: is dead neutrophil, necrotic tissue, dead
macrophages tissue fluid. A localized collection of pus
is called abscess.
B- Acquired or adaptive immunity: substances that
activate adaptive immunity are called antigens which
are large molecules with a molecular weight of 10000 or
more for example penicillin.
There are two types of antigens:
a. foreign antigens: Pollen, food, drugs and dried
Transplanted organs and tissues that contain foreign
antigens result in the rejection of the transplant. b.Selfantigens: that produced by the body itself. There are
two types of acquired immunity:
1. Humoral immunity.
2. Cellular immunity.
Humoral immunity: is mediated by B-lymphocytes
which are produce plasma cells.Plasma cell produce
circulating antibodies or immunoglobulins (Ig,s).
Humoral immunity can also cause immediate
hypersensitivity reactions such as bronchial asthma.
Cellular Immunity: is mediated by T-lymphocytes.
It is responsible for delayed allergic reactions (such as
eczema) and rejection of transplants of foreign tissue.
The cytotoxic T cells destroy cells that have the antigen
which activated them. One mechanism by which they
kill is insertion of pore-forming molecules (perforins)
in the membranes of their target cells which leads to
destruction of the cells by osmotic lyses.
Immune system problems of clinical significance:
Hypersensitivity (allergy): is an undesirable side effect of
immunity. It is defined as an inappropriate activation of the
body’s immune system which may result in a very strong
inflammation and tissue damage. Types:
1.Immediate hypersensitivity: which is caused by B-cell
immunity. There is reaction between antibodies and antigens.
The symptoms appears within a few minutes of exposure to
foreign antigen. For example, bronchial asthma, food
allergies, hay fever.
2.Delayed hypersensitivity: mediated by T-lymphocytes and
symptoms take several hours or days to develop. Examples,
poison oak, eczema, soaps, measles, cosmetics, & drugs.
Q: write short notes on the allergies in the allergic
person. Give examples.

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