Immunity is the capability of multicellular
organisms to
resist harmful microorganisms from
entering it. Immunity involves both
specific and nonspecific components. The
nonspecific components act as barriers
or eliminators of a wide range of
pathogens irrespective of their
antigenic make-up. Other components of
the immune
system adapt
themselves to each new disease
encountered and can generate
pathogen-specific immunity.
An immune system may contain innate and adaptive components.
The innate system in mammalians, for
example, is composed of primitive bone
marrow cells
that are programmed to recognise foreign
substances and to react. The adaptive
system is composed of more advanced lymphatic cells
that are programmed to recognise
self-substances and not to react. The
reaction to foreign substances is
etymologically described as inflammation,
meaning to set on fire. The non-reaction
to self-substances is described as
immunity, meaning to exempt or as
immunotolerance. These two components of
the immune system create a dynamic
biological environment where "health"
can be seen as a physical state where
the self is immunologically spared, and
what is foreign is inflammatorily and
immunologically eliminated. "Disease"
can arise when what is foreign cannot be
eliminated or what is self is not
spared.
Innate immunity, also called native
immunity, exists by virtue of an
organisms constitution, that is its
genetic make-up, without an external
stimulation or a previous infection. It
is divided into two types: (a)
Non-Specific innate immunity, a degree
of resistance to all infections in
general. (b) Specific innate immunity, a
resistance to a particular kind of
microorganism only. As a result, some
races, particular individuals or breeds
in agriculture do not suffer from
certain infectious diseases.
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Adaptive immunity can be sub-divided
depending on how the immunity was
introduced in 'naturally acquired'
through chance contact with a
disease-causing agent, whereas
'artificially acquired immunity'
develops through deliberate actions such
as vaccination. Both naturally and
artificially acquired immunity can be
further subdivided depending on whether
the host built up immunity itself by
antigen as 'active immunity' and lasts
long-term, sometimes lifelong. 'Passive
immunity' is acquired through transfer
(injection or infusion) of antibodies or
activated T-cells from an immune host;
it is short lived—usually lasting only a
few months. The diagram below summarizes
these divisions of immunity.
Adaptive immunity can also be divided by
the type of immune mediators involved; humoral
immunity is
the aspect of immunity that is mediated
by secreted antibodies, whereas cell
mediated immunity involves
T-lymphocytes alone. Humoral immunity is
called active when the organism
generates its antibodies, and passive
when antibodies are transferred between
individuals or species. Similarly,
cell-mediated immunity is active when
the organisms’ T-cells are stimulated,
and passive when T cells come from
another organism
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