Types of Immunity (OCR A Level Biology)

Active immunity

• Active immunity is acquired when an antigen enters the body triggering a specific immune response (antibodies are produced)
• Active immunity is naturally acquired through exposure to microbes or artificially acquired through vaccinations
• The body produces memory cells, along with plasma cells, in both types of active immunity giving the person long-term immunity
• In active immunity, during the primary response to a pathogen (natural) or to a vaccination (artificial), the antibody concentration in the blood takes one to two weeks to increase.
• If the body is invaded by the same pathogen again or by the pathogen that the person was vaccinated against then, during the secondary response, the antibody concentration in the blood takes a much shorter period of time to increase and is higher than after the vaccination or first infection

The primary and secondary response to the same antigen

Passive immunity

• Passive immunity is acquired without an immune response. Antibodies are not produced by the infected person
• As the person’s immune system has not been activated then there are no memory cells that can produce antibodies in a secondary response. If a person is reinfected they would need another infusion of antibodies
• Depending on the disease a person is infected with (eg. tetanus) they may not have time to actively acquire the immunity, that is, there is no time for active immunity. So passive immunity occurs either artificially or naturally
• Artificial passive immunity occurs when people are given an injection / transfusion of the antibodies. In the case of tetanus this is an antitoxin. The antibodies were collected from people whose immune system had been triggered by a vaccination to produce tetanus antibodies
• Natural passive immunity occurs when:
  • Foetuses receive antibodies across the placenta from their mothers
  • Babies receive the initial breast milk from mothers (the colostrum) which delivers a certain isotype of antibody (IgA)

Comparing Active & Passive Immunity Table