Acquired Immunity


Immunity can be described as either passive or active, depending on how it is acquired:

  • Active immunity is due to the production of antibodies by the organism itself after the body's defence mechanisms are stimulated by antigens
  • Passive immunity results from the acquisition of antibodies from another organism in which active immunity has been stimulated

Both types of immunity can also be described as natural or artificial, depending on whether the acquisition of antibodies was natural or induced

  • Natural Active Immunity:  Producing antibodies in response to pathogenic infection (i.e. challenge and response)
  • Artificial Active Immunity:  Producing antibodies in response to a controlled exposure to an attenuated pathogen (i.e. vaccination)
  • Natural Passive Immunity:  Receiving antibodies from an external source naturally (e.g. to fetus via colostrum and to newborn via breast milk)
  • Artificial Passive Immunity:  Receiving antibodies from an external source via injection (e.g. blood transfusion of monoclonal antibodies)


  • Vaccinations induce artificial active immunity by stimulating the production of memory cells 
  • A vaccine contains weakened or attenuated forms of the pathogen and is (usually) injected into the bloodstream
  • Because a modified form of the pathogen is injected, the individual should not develop disease symptoms
  • The body responds to the vaccine by initiating a primary immune response, resulting in the production of memory cells
  • When exposed to the actual pathogen, the memory cells trigger a secondary immune response that is much faster and stronger
  • Vaccines confer long-term immunity, however because memory cells may not survive a life time, booster shots may be required


  • Vaccination results in active immunity
  • It can limit the spread of infectious diseases (pandemics / epidemics)
  • Diseases may be eradicated entirely (e.g. smallpox)
  • Vaccination programs may reduce the mortality rate of a disease as well as protect vulnerable groups (e.g. youth, elderly)
  • Vaccinations will decrease the crippling effects of certain diseases (e.g. polio) 
  • It will decrease health care costs associated with treating disease conditions


  • Vaccinated individuals may produce (mild) symptoms of the disease
  • There may be human error in the preparation, storage or administration of the vaccine
  • Individuals may react badly to vaccines (e.g. hypersensitive / allergic reactions)
  • Immunity may not be life long - booster shots may be required
  • There may be possible toxic effects of mercury-based preservatives used in vaccines

Monoclonal Antibodies

  • Monoclonal antibodies (mAb) are antibodies derived from a single B cell clone and are an example of artificial passive immunity
  • An animal (typically a mouse) is injected with an antigen and produces specific plasma cells
  • The plasma cells are removed and fused (hybridised) with tumor cells capable of endless divisions (immortal cell line)
  • The resulting hybridoma is capable of synthesising large quantities of specific antigen, for use in diagnosis and treatment

Production of Monoclonal Antibodies

Treatment Use:

  • Monoclonal antibodies can be used for the emergency treatment of rabies
  • Because the rabies virus is potentially fatal in non-vaccinated individuals, injecting purified quantities of antibody is an effective emergency treatment for a very serious viral infection

Antimicrobial Agents

  • In addition to providing immunity via the acquisition of antibodies, diseases may be treated with antimicrobial agents (i.e. pharmaceutical drugs) that specifically target particular pathogens
  • Antibiotics are substances or compounds that kill or inhibit the growth of bacteria by targeting the metabolic pathways of prokaryotes
  • Specific prokaryotic features that may be targeted by antibiotics include key enzymes, 70S ribosomes and the bacterial cell wall
  • Because eukaryotic cells do not have these features, antibiotic can kill bacterial cells without harming humans
  • Antibiotic agents can be categorised according to their specificity as either broad spectrum or narrow spectrum
    • Broad spectrum antibiotics are effective against a wide variety of bacteria
    • Narrow spectrum antibiotics are very specific against certain bacteria and of limited use in a wider context
  • Antibiotics may also be described as bacteriostatic or bacteriocidal according to their effect
    • Bacteriostatic drugs inhibit bacterial growth but do not kill the bacteria (good for allowing immunity to develop)
    • Bacteriocidal drugs kill the bacteria (good for particularly dangerous strains of  bacteria)

Bacteriostatic versus Bacteriocidal Agents

  • Virus do not carry out metabolic reactions themselves but instead infect host cells and take over their cellular machinery
  • Because viruses do not have metabolism, antibiotic agents are not effective against viruses
  • Viruses need to be treated with specific antiviral agents that target features specific to viruses (e.g. reverse transcriptase in retroviruses)
  • Similarly, fungal infections and helminth infections can be treated with agents that target fungal cell features and helminth features respectively