The overall ability of the host to fight the disease causing organism conferred by the immuni system is called immunity.

Immunity is of two types:

(i) Innate immunity

(ii) Acquired immunity.

Innate immunity:

Innate immunity is non-specific type of defence, that is present at the time of birth. Innate immunity consists of four types of barriers.

(i) Physical barriers: Skin on our body is the main farrier which prevents entry of the micro-organisms.

(ii) Physiologyical barriers: Acid in the stomach, saliva in the mouth, tears from eyes, all prevent microbial growth.

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

(iv) Cytokine barriers: Virus infected cells secrete protein called interferenos which protect non-infected cells from further viral infection.