11.3 - Types of Immunity

Active immunity means the body's immune system is actively involved in the production of antibodies. In other words, they are not obtained from external sources, for example, through injections. On the contrary, passive immunity means the immune system is not actively involved in antibody production. The antibodies are received from an external source such as antiserum or across the placenta from mother to fetus.

Now, both of these can be further divided into natural and artificial. Natural simply means what triggers the production of antibodies is something naturally-occurring or the immunity is naturally existing. Artificial refers to synthesized substances being introduced to a person.

Figure 17 (left) - When someone catches a flu, he/she is exposed to the flu virus which in turn stimulates the production of antibodies to remove the viruses. Figure 18 (right). Vaccination introduces dead or weakened disease-causing pathogens into the body which will trigger the immune system to produce antibodies.

Figure 18 (left) - During pregnancy, maternal antibodies can move across the placenta to the fetus for protection (IgG is the smallest antibody and therefore can easily diffuse through the membrane). After birth, the baby continues to acquire antibodies (mainly IgA) from the mother through breastfeeding. Figure 20 (right). Normally, artificial immunization is given when a person is going to travel to a country with high risk of certain disease such as hepatitis A or as an emergency treatment for rabies, tetanus or during an outbreak.

Similarities and differences between active and passive immunity

• Similarities

  • Outcome Principles
    • Keep people safe from communicable diseases.
    • Involves antigen-antibody interaction.

• Differences

  • Active Immunity
    • Achieved through:
      • Active production by the host's immune system
      • Induced natural through infections or injection of vaccines
      • Vaccine contains dead or weakened pathogens
    • Effect: Does not provide immediate protection. Takes time for the effect to be seen.
    • Antibody: Antibodies are produced by the body's lymphocytes
    • Memory: Immunological memory present
    • Duration of protection: Life-time and effective protection
    • Booster dose: Booster dose is required in artificial active immunity to raise the amount of antibodies above the immunity level
  • Passive immunity
    • Achieved through:
      • Receive passively by the host. The host's immune system is not actively involved in the production of antibodies.
      • Administration of ready-made antibodies (antiserum)
      • Antiserum contains specific antibodies against specific pathogens
    • Effect: Provide immediate protection to host
    • Antibody: Antibodies are obtained from antiserum
    • Memory: No memory
    • Duration of protection: Short-term and less effective protection
    • Booster dose: In artificial passive immunity, a booster dose is only needed when the amount of antibodies drop below the immunity level or the person is still infected.

Graph 1 - This illustrates how the level of antibodies changes during time in active immunity. The first exposure to the pathogen or vaccine will stimulate the first immune response. Note that the second exposure of the same pathogen at a later time stimulates a higher level of antibodies production due to the presence of immunological memory. As the body is actively involved in the production of the antibodies, the presence of memory cells help the body remember how to respond to the pathogens previously encountered. The time taken for the second antibodies production will also be shorter, which means the body can respond faster to destroy the pathogens. The level of antibodies stays high above the immunity level after each encounter with the pathogen.

Graph 2 - In artificial passive immunity, note that the level of antibodies shoots up spontaneously after injection because the antibodies are introduced directly into the bloodstream. The body is not actively involved in the production and hence no memory cells are produced. After elimination of the pathogens, the antibodies dropped rapidly below immunity level, which means the person is no longer immune to the disease. Only after the second injection of the antiserum, will the person be protected from getting the disease.