We are constantly being exposed to infectious
agents and yet, in most cases, we are able to resist these infections.
It is our immune system that enables us to resist infections. The immune
system is composed of two major subdivisions, the innate or nonspecific
immune system and the adaptive or specific immune system. The innate
immune system is our first line of defense against invading organisms
while the adaptive immune system acts as a second line of defense and
also affords protection against re-exposure to the same pathogens.
‘Natural immunity’ of a living being
is also known as innate immunity, native immunity or inherited immunity.
This relates to a general or non-specific type of resistance, which
prevents infection by different kinds of pathogens. The extent of this
natural immunity differs in different organisms.
For example, the viruses that cause leukemia
in cats or distemper in dogs don’t affect humans. Innate immunity
works both ways because some viruses that make humans ill — such
as the virus that causes HIV/AIDS — don’t make cats or dogs
Innate immunity also includes the external barriers
of the body, like the skin and mucous membranes (like those that line
the nose, throat, and gastrointestinal tract), which are our first line
of defense in preventing diseases from entering the body. If this outer
defensive wall is broken by an injury, the skin attempts to heal the
break quickly and special immune cells on the skin attack invading germs
by inflammatory response and phagocytosis. Antiviral proteins like interferon
are also secreted by the white blood cells.
‘Acquired immunity’, develops during
the lifetime of an individual and refers to the immunity, which specific
individual displays against a specific pathogen. This is frequently
related to the presence of antibodies or interferon in the blood. Acquired
immunity based on antibodies is the most efficient type of acquired
immunity and it may be either
(i) Actively acquired (or)
(ii) Passively acquired.
Actively Acquired Immunity - ‘Actively acquired immunity’
may be either natural or artificial. Actively acquired natural immunity
results from any infection from which a person recovers.
During the infection, antibody production for
that specific pathogen is stimulated so that when there is a subsequent
infection by either the same or antigenically related pathogen, the
antibodies assist in the body’s defense.
Actively acquired artificial immunity is the
most common method of immunization or vaccination. The immunogens are
injected in the body in controlled quantity to stimulate the production
of immunoglobulins. Killed and attenuated strains of bacteria and viruses
are now used widely for immunization against many diseases like typhoid,
small pox, poliomyelitis, yellow fever, measles, etc. Attenuated organisms
produce mild infection and induce natural immunity. Vaccines are also
being developed now by a variety of other methods involving recombinant
Haptens are small foreign molecules that cannot
by themselves induce antibody formation. However, if attached to macromolecules
they can elicit the formation of a specific antibody. The attached molecule
is called a haptenic determinant, and the molecule to which it is attached
is called a carrier. Some medicinal properties are very effective in
eliciting the formation of antibodies, and are used experimentally as
a haptenic determinant. Homoeopathic medicines act like haptens.
Passively Acquired Immunity
‘Passive immunity’ may also be acquired
either by natural or artificial means. While passive immunity acquired
by natural means, involves the transfer of antibodies from mother to
her unborn child, through the placenta during the later part of pregnancy,
passive immunity of the artificial type refers to the original production
of antibodies in some other individual (human or lower mammal) followed
by injection of these antibodies with the help of a needle or syringe.
Several drug-manufacturing companies are involved
in the large-scale production of antibodies in horses and cows by active