Concept white blood cells (leukocytes), the major phagocytes are

Concept of Immmunity

The Body Defenses as per modern
science is termed as Immunity which can be regarded as resistance to disease.  the immune system is responsible for defense
against all the microorganisms and toxic cells to which  individuals are exposed.  There are many factors which directly or
indirectly affect the immunity eg. Genetics, Age, Health, nutrition, stress,
Hormones etc. The immune system is a functional system rather than a system
with discrete organs. The parts of almost all organs in the body play some role
in immunity. Wide spread chemical mediators, cells and tissues along with their
transport via  circulatory and lymphatic
systems constitute the immune system. There are mainly two types of immunity
present in body. First one is  1.
Nonspecific Immunity an innate reaction that acts as a general response against
all kinds of pathogens without having been previously exposed to it. The
examples of non specific immunity are  a.
physical and chemical barriers b. internal cells and chemicals 2. Specific
Immunity an adaptive system that fights specific individual pathogens in
customized and professional ways

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Cells and tissues of the immune system

The cells of the immune system take
their origin from precursors in the bone marrow, after which they circulate in
the blood and live in lymphoid organs like lymph nodes, spleen, tonsils along
with virtually within all the tissues of the body. During the demand they can
rapidly migrate to any site of infection. Among circulating white blood cells
(leukocytes), the major phagocytes are neutrophils and monocytes. These cells
ingest and destroy microbes, other injurious agents, and one’s own dead and
damaged cells. Neutrophils respond rapidly to foreign stimuli and to injury;
their reaction is part of acute inflammation. When blood monocytes enter
tissues, they mature, and are called macrophages. These cells are present under
epithelia, in connective tissues, and in all organs. Macrophages respond more
slowly than do neutrophils but for longer times; this reaction is typical of
chronic inflammation. Macrophages also help to repair damaged tissue. The most
important cells of adaptive immunity are lymphocytes which are catagorised in
two main classes B lymphocyte and T lymphocyte. B lymphocytes (so called
because they mature in the bone marrow) secrete proteins called antibodies,
which bind to and eliminate extracellular microbes. T lymphocytes (which mature
in the thymus) function mainly to combat microbes that have learned to live
inside cells (where they are inaccessible to antibodies). There are two main
types of T lymphocytes one is helper T cells which help B lymphocytes to make
the most effective antibodies and help macrophages to kill ingested microbes.
The second group of T lymphocyte are called cytotoxic (cytolytic) T lymphocytes
(CTLs) which kill infected host cells and thus serve to eliminate reservoirs of
infection. A third, small population of lymphocytes is called “regulatory T
cells” because they control immune responses and prevent inappropriate
reactions. There are several other small populations of lymphocytes. In order
to get immune responses started, foreign substances have to be captured and
displayed to lymphocytes. The cells that perform this task of displaying
antigens are called antigen-presenting cells (APCs). The best defined APCs are
specialized type of cells called dendritic cells. 


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