“In the struggle for survival, the fittest win out at the expense of their rivals because they succeed in adapting themselves best to their environment”. – Charles Darwin

All living beings follow survival of the fittest and struggle for existence laws of Charles Darwin at all levels of evolution.

Microbes which existed long before humans on our planet are the best examples for the statements of Darwin. For their survival microbes obtained various adoptive modification, parasitism is one of their modifications. The parasitic modifications of microbes posed great threat to the life of the remaining organisms.

As a part of their struggle for existence higher animals acquired a defense system known as “immune system”. It is made up of a network of cells, tissues, and organs that work together to protect the body.

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In biology a separate branch “immunology” has been developed to study and understand this natural defensive mechanism. The term immunology has been derived from the Latin term “immunus”, which means “freedom from” or “exempt”.

Immunity provided by immune system is the state of protection from infections and diseases causing agents. Birth of the subject immunology took place in the 18th century through the efforts of an English physician Edward Jenner, when he tried to highlight the protective role of vaccine against smallpox disease.

Louis Pasteur’s accidental discovery of techniques that can minimize the virulence of pathogenic microbes strengthened the roots of immunology.

Immunity

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The extraordinary coordination of different cells, proteins, tissues, and organs of immune system provide immunity to the body against infectious organisms and other invaders. Blood and lymph systems play a vital role in coordinating the function of immune system.

Since the pathogens that can cause disease arrive from different parts of the biological kingdom, immune’ systems response is not similar every time. This response depends upon various factors such as mode of entry of pathogen, type of pathogen, behavior of pathogen etc.

In spite of the surveillance done by the immune system, certain pathogens manage to enter into the body and cause diseases.

Skin and mucosa are general possible pathways for the entry of pathogens.