Immunity is the state of resistance exhibited by the host body against any foreign particle or microorganisms. This resistance plays a major role in the prevention of infectious diseases.



  • Infants and aged people have low immunity
  • Diabetics
  • Burn patients
  • Pregnant women
  • Chemotherapy and radiotherapy
  • Persons consuming antibiotics has low immunity

Factors like a skin (mechanical barrier), mucosa of the respiratory tract, saliva (in the mouth), Tears (contains lysozyme) etc. provides immunity to the body.


⇒ Immunity has been classified into two groups as follows –

  • Innate immunity
  • Acquired immunity

Innate Immunity

⇒ It provides the first line of defense against infections.

⇒ Innate immunity is the natural immunity, which is passed genetically from one generation to other generation.

⇒ It does not depend on prior contact with the microorganism.

⇒ It is present in an individual by birth.

⇒ Innate immunity is of two types as follows –

  • Specific innate immunity
  • Non-specific innate immunity

The Specific Innate Immunity is where resistance to a particular pathogen is concerned. This type of immunity immunizes the individual for a specific single pathogen. On the other hand, the Non-specific Innate Immunity shows a degree of resistance to infections in general.

Specific innate immunity have been further classified as –

  • Species innate immunity
  • Racial innate immunity
  • Individual innate immunity

In Species innate immunity individuals of the same species show the certain degree of resistance towards a particular disease, for example, Salmonella typhi produces Typhoid fever in Man whereas mice are resistant.

In Racial innate immunity within a species different races show different levels of immunity for the same infection this depends upon the type of diet, way of living etc., for example, Negros are resistant to yellow fever and malaria.

In Individual innate immunity, individuals in a population show variation in their response to microbial infection. Every individual show different states of resistance towards pathogens, for example, during epidemics, different members of a family or a community show differences in the level of immunity. Some will develop mild why some will totally escape from the disease. This is due to individual immunity. It is influenced by age hormonal secretions and nutrition etc.

Acquired immunity

⇒ Acquired immunity is the immunity acquired during the lifetime of an individual. It is not inherent in the body by birth.  It is specific to a single type of microorganism.

⇒ Acquired immunity is of two types –

  • Active acquired immunity 
  • Passive acquired immunity 

Active acquired immunity –

⇒ Active acquired immunity is immunity developed as a result of the antigenic stimulus.

⇒ The host’s body takes an active part in the production of antibodies.

⇒ It lasts for a longer period.

⇒ It is also known as adaptive immunity as it represents the adaptive response of the host to a specific pathogen or other antigens.

⇒ Acquired immunity is of two types:-

  • Natural active acquired immunity
  • Artificial active acquired immunity

Natural active acquired immunity is the immunity acquired by natural infection of the organism. For e.g. a person who has recovered from an attack of measles develops natural acquired immunity.

Artificial active acquired immunity is the immunity acquired by the vaccination is called as artificial active acquired immunity. Vaccines are the products of live or killed or weakened (attenuated) microorganisms or their products used for immunization. For e.g. BCG vaccine, Sabin vaccine, Salk vaccine etc.

Passive acquired immunity –

⇒ The immunity received passively by the host is termed as passive acquired immunity.

⇒ There is no antigenic stimulus; instead preformed antibodies are administered, which were produced in experimental animals like Rabbit & Horse.

⇒ It is less effective than the active immunization

⇒ It is again of two types –

  • Natural passive acquired immunity
  • Artificial passive acquired immunity

Natural passive acquired immunity is the immunity developed in an individual passively by the external natural source like the mother to fetus and from colostrum to infants.

Artificial passive acquired immunity is the immunity developed in individual passively but produce externally by experimental animal – which produces antibodies actively against the antigen and thus produced antibodies are injected in the individual who is unable to produce antibodies by himself.

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