The mechanism used by the body as protection against environmental agents that foreign to the body. Immunity is a state of protection against foreign pathogens or substances (antigens).

It responds to threats on an individual basis.

Term immunity derived from – Immunis, meaning Exempt.

Immune system is remarkably versatile defense system that has evolved to protect animals from invading pathogenic microorganisms and cancer.

Types of Immunity:

  • Innate immunity: primitive immune system which is less specific
  • Adaptive immunity: highly evolved system of specific response

Innate Immunity

  • Also called Native
  • Resistance possessed by an individual by birth i.e., inherited
  • Provides first line of defense against infections
  • Genetically determined: no prior exposure or antibody production involed
  • 3 levels:

Species immunity: entire human species is resistance to plant pathogen.

Racial immunity: particular races of that species are resistant. Example: Negroid species in USA is more prone to TB than causcian species.

Individual immunity: evident on twin – smiliar degree of resistance to TB or leprosy.

  • They are always present and available at short notice to protect the individual from challenges by foreign invaders
  • Elements of innate immune system includes body surfaces and internal components, such as: skin, the mucous membrane, cough reflex.
  • It is carried out by non-specific physical and chemical barriers (eg-skin), cellular barriers (eg-phagocytes) and molecular pattern based reactions (eg-toll like receptors or TLRs).
  • It is line of defense:

First line of defense: intact skin and mucosae prevent entry of microorganism

Second line of defense: antimicrobial proteins, phagocytes and other cells.

  • Factors influencing the level of innate immunity:
  • Age
  • Very old or very young, more susceptible to infectious disease.
  • In foetus, immune system is immature where as in old age there is gradual decrease in immune system
  • Hormones
  • Endocrine disorders such as Diabetes mellitus, Hypothyroidism, and Adrenal dysfunctions
  • Staphylococcal sepsis is more common in Diabetes, which may be caused by increased level of carbohydrates in tissues.
  • Nutrition
  • Immune system is reduced in malnutrition patients
  • In Kwashiorker ( serve protein deficiency system), cell mediated system reduces.

Acquired or adaptive Immunity

  • The resistance that human acquires during life
  • Provides second line of defence
  • Produces by prior exposure or antibody production
  • Characteristics:

Antigen specificity: immune system or antibiotics can distinguish among antigens, even between 2 proteins that differ in only 1 amino acid.

Diversity: immune system is capable of generating large antibody diversity in its recognition molecules.

Immunologic memory: immune system exhibits memory on second encounter of same antigen by generating a second response which is more specific and quick.

Self and non self recognition: does not react with body’s own molecule but effectively eliminates foreign antigens.

  • Types:

Active: natural active immunity and artificial active immunity

Passive: natural passive immunity and artificial passive immunity

Active immunity

  • also called acquired immunity
  • Resistance developed by an individual as a result of an antigenic stimulus.
  • Used for prophylaxis to increase body resistance.
  • Produced by antibodies that develop in response to antigens (Immune response)
  • Latent period: requires time for its activation.
  • Negative phase: a time period in which there is reduced measurable immunity – antigen combines with the pre-existing antibody and lower its circulation.
  • More effective and better protection than passive.


Natural active immunity:

  1. Clinical infection by a microbe
  2. Develops after exposure to antigens in environment
  3. Long lasting immunity but varies on type of pathogens
  4. Large majority of adult – active immunity against Poliovirus
  5. Life long immunity – viral diseases like Chicken pox or Measles
  6. Premuntion: immunity to re-infection last long till the disease remains active.
  7. Example: Typhoid.

Artificial active immunity:

  1. Resistance reduced by vaccines
  2. May be either bacterial or viral vaccines
  3. Develops after administration of antigen to prevent disease
  4. Booster dose: repeated dose of vaccines following the primary vaccination prolong the immunity
  5. May be either in form of:
  6. Live immunity: initiates the infection without causing any injury and disease. It parallels with natural but of low grade. Example: BCG for tuberculosis.
  7. Killed vaccines: comparatively less immunogenic. Protection lasts only for a short period. Have to be supplemented by booster dose repeatedly. May be given orally. Example: Cholera vaccine.
  8. Bacterial products: example: Tetanus toxoids.

Passive Immunity

  • Resistance transmitted passively to a receipent in a readymade form
  • Produced by transfer of antibodies from another person
  • Receipents immune system plays no active role
  • Used for the treatment of acute infection


Natural passive immunity:

  1. Conferred by transfer of maternal antibodies across placenta or in breast milk
  2. Resistance passively transferred from mother to foetus or infants, through placenta and through milk (colostrums – IgA antibodies)
  3. Human fetus – ability to synthesize IgM from 20th week of life
  4. Transport of antibodies across the placenta is an active process – concentration in fetal blood is higher than in mother blood.

Artificial passive immunity:

  1. Resistance passively transferred to a recipient by administration of antibodies.
  2. Conferred by administration of antibodies to combat infection
  3. Agents used: hyperimmune sera of animal, covalescent sera, pooled human gamma globulin
  4. These are used for psophylaxis and therapy.