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Immunity is the
body’s self-defense mechanism against diseases. Pollutants and germs can enter
the human body through skin, mucus membranes (eyes, nose, urinary or genital
tracts), food and water, as well as the air that we breathe. The Human Defense
system works at two levels. That is local defense mechanism, which is
prevention of entry of germs, and the immune system. The primary local defense
system tries to form mechanical barriers, thus preventing the entry of germs.
The Immune system fights the germs after they have entered into the body

The main benefits
of the local defense system are its quick response and power to fight against a
wide range of potentially infectious threats. The parts of the local defense
system are independent of previous exposure to infections. The local defense system
consists of: protective mechanical barriers; germs or foreign bodies thrown out
if entered; germ killing secretions; germ fighting white blood cells.

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Skin, hair and
mucus form primary mechanical barriers of a human body. Skin is made of the
protein keratin, it forms a protective covering on the body and is impermeable
to germs, so the skin is a very important first line of defense against foreign
invaders. Any cut or injury on the skin opens the way for germs to invade the
body. Mucus is the slimy section of the epithelial cells that forms a delicate
covering of many organs. The mucous present inside the windpipe and the nasal
chamber traps the germs. Even if germs enter the respiratory or the digestive
systems, they are thrown out by vomiting, coughing and sneezing. Sometimes lose
motions of diarrhea also help to clear the digestive tract of the germs.

Saliva, sweat
tears and nasal secretions contained several germs killing secretions.

The hydrochloric
acid secreted by the stomach, kills the germs that enter our body through the

The white blood
cells or phagocytes are the cells of the immune system. They fight with the
foreign materials and prevent the body from infections. The white blood cells
squeeze out the blood capillaries by the process of “diapedesis” and destroy
the germs. This process is known as phagocytosis.

The local defense
system may not be effective at all times, in certain cases the microbes invade
all the mechanical barriers and enter the deeper tissues and organ systems.
Under such circumstances the human body produces special proteins: antibodies
and antitoxins to fight against the threat. In other words, “Immunity” is
defined as the capacity of our body to deal with foreign substances as
bacteria, viruses, toxins, etc. that enter our body and the render them
harmless. The breaching of the skin, for example by a splinter, could provide a
route for bacteria to enter the body. These invading bacteria are regarded as
non-self substances nu the body’s immune system, thus triggering an
inflammatory response in an attempt to destroy the invading agents.

A special type of
white blood cells, known as a macrophage takes important role in first defense
step. Macrophage engulfs the non-self bacterium reducing it to tiny remnants,
known as antigens; which are displayed on the external surface of the
macrophage, and also serving as an antigen presenting cell (APC). This alerts
the body’s other macrophages, to the presence of the bacteria in the body. The
macrophage also communicates with antigen presenting cells by releasing
cytokines which fit perfectly into the receptors of the cells. So, this results
in the increase in number of macrophages at site of the inflammation. Additionally,
some of the normal cells also function as antigen presenting cells by ingesting
some of the bacteria and covering self surface with the antigens. This type of
immune response is immediate and nonspecific.

Immunity can be
innate or acquired. Innate immunity (also called natural or innate immunity) is
by virtue of the genetic constitutional make-up of the body. It is there in the
body without any external simulation or previous infection.

Innate immunity
can be nonspecific and specific. Specific immunity is the natural resistance
against a specific germ only. For example, human beings are immune to a highly
infectious disease of dogs known as distemper, which kills about 50% of all
infested dogs. Non-specific immunity is natural resistance to all kind of
infections in general. For example: human beings are immune to highly
infectious diseases of plants. Acquired Immunity is the resistance which an
individual acquires during his or her life-time.

This immunity is
of two types, namely actively acquired immunity and passively acquired
immunity. Actively acquired immunity is the resistance developed due to a
previous infection or due to an antigen. Antigen is the chemical found on the
surface of the disease-causing germ cell. If the antigen is introduced in the
body by artificial means during vaccination, it can lead to an artificially
acquired active immunity. On the other hand if the antigen enters his body
naturally, it is called naturally acquired active immunity. This kind of
resistance is possible due to some previous infection or entrance of antigen into
our body naturally. In either of the cases the body’s lymphocytes react by
producing antibodies or killer cells. The produced antibodies circulate in the
blood and the lymph, thereby killing the microbes. Passively acquired immunity
is the introduction of ready-made antibodies from an outside source. It is of
two types.

Naturally acquired
passive immunity: in this type of defense mechanism, the prepared antibodies of
the mother, enters the foetus through the placenta.

Acquired Passive Immunity: in this, the antibodies are extracted from the blood
of some animals after injecting germs into their bodies. The serum containing
the antibodies is collected and used to make antiserum injections like
antivenin or anti-diphtheria. These injections are then injected into the
victim’s body. For example in the treatment of snake-bite, antivenin injections
are given to the patient.

Antibodies are
special chemicals found in the blood that fight germs or their secretions. The
specialized lymphocytes on exposure to antigens produce antibodies. These
specialized lymphocytes are present in the lymph nodes, spleen, lymph and in
the circulating blood. Antibodies are proteins belonging to the class of
immunoglobulin. Antibodies are specific as one kind of antibody acts only
against one type of an antigen. An antibody after recognizing its antigen binds
with it and renders it harmless which is later destroyed and eliminated by the

Immunity produced
by antibodies can be for a short period as in common cold cholera, or for a
longer period as in smallpox, measles. Some of the antibodies are present in
the blood of some people from birth. These people are immunized against certain
even if the germs have invaded the body. 

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