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 tissues.
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 consist of: Protective
mechanical barriers; germs or foreign bodies thrown out if entered; germ
killing secretions; germ fighting white blood cells.
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. 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 food.
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
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. 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 specific and
nonspecific. 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