What is Immunity?

The overall ability of the host to fight disease causing organisms, conferred by the immune system is called Immunity.

- Immunity is of two types : $(i)$ Innate immunity $(ii)$ Acquired immunity.

$(i)$ Innate immunity : It is a non-specific type of defence, that is present at the time of birth.

This is accomplished by providing different types of barriers to the entry of foreign agents into our body. Innate immunity consist of four types of barriers.

Physical barriers : skin on our body is the main barrier which prevents entry of the microorganisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.

Physiological barriers : Acid in stomach, saliva in the mouth, tears from eyes all prevent microbial growth.

Cellular barriers : Certain types of leukocytes(WBC) of our body like polymorpho-nuclear leukocytes(PMNL -neutrophils) and monocytes and natural killer(types of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.

Cytokine barriers : Virus infected cells secrete proteins called interferons which protects non infected cells from further viral infection.

$(ii)$ Acquired immunity : Acquired immunity is pathogen specific. It is characterised by memory.

Our body when encounters a pathogen for the first time produces a response called primary response.

Primary response is of low intensity.

Subsequent encounters with the same pathogen elicits a highly intensified secondary or anamnestic response.

This is ascribed to the fact that our body appears to have memory of the first encounter.