Class 12 Biology Human Health Disease Immunity


The overall ability of the body to fight against disease causing microorganisms with the help of immune system is called immunity.

Immunity is of two types-

1. Innate immunity.

2. Acquired immunity.

Innate immunity

The immunity which occurs by birth is called innate immunity.

Innate immunity is non-specific type of defense.

Innate immunity consists of various barriers which prevent the entry of microorganisms into the body.

Innate immunity consists of four types of barriers.

  • Physical barriers-Skin on our body is the main barrier which prevents entry of the micro-organisms and mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
  • Physiological barriers- Acid in the 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), monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.
  • Cytokine barriers- Virus-infected cells secrete proteins calledinterferons which protect non-infected cells from further viral infection.

Acquired immunity

The immunity which develops during lifetime by exposure to suitable foreign agents like microorganisms is called acquired immunity.

Acquired immunity is pathogen specific and it ischaracterized by memory.

When the body first encounters a pathogen it produces a response which takes long time to develop and of low intensity called as primary immune responseand if the body encounters the same pathogen it produces highly intensified and quick response called as secondary immune response.

After responding to the foreign microorganisms and elimination of the pthogen, the immune system keeps the memory of the that encounter (primary response) and during its second encounter with the same pathogen produces a highly intensified immune response (secondary response).

The primary and secondaryimmune responses are carriedout with the help of twospecial types of lymphocytes present in our blood,

  • B-lymphocytes :The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them called as antibodies
  • T-lymphocytes :  The T-cells themselves do not produce antibodies but help B cells to produce them

Fig. B-lymphocyte and T-lymphocyte  

Share these Notes with your friends  

< Prev Next >

You can check our 5-step learning process