Immunity:

The ability of the host to fight the disease-causing organisms through immune system is called immunity.
Immunity is of two types:

1) Innate immunity:

• It is non-specific, present from birth and inherited from parents.
• Achieved by providing various barriers:

a) Physical barriers: Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
b) Physiological barriers: Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth.
c) Cellular barriers: Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.
4) Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

2) Acquired immunity:

• It is pathogen specific, acquired after birth.
• It is characterised by memory. This means that our body when it encounters a pathogen for the first time produces a response called primary response which is of low intensity.
• Further encounter with the same pathogen leaves a highly intensified secondary or anamnestic (memorial) response.
• The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., B-lymphocytes and T-lymphocytes.
• The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies.
•  The T-cells themselves do not secrete antibodies but help B cells to produce them.
• Two types of acquired immune responses are produced in our body:
• (a) Humaral immune response: It is mediated by antibody. So, called antibody mediated immunity also.
•  (b)Cell mediated immunity: It is mediated by T-lymphocytes and responsible for graft rejection in case of organ transplant mismatch.
• Acquired immunity is further of two types: Active and passive.