Making Your Own Defence: Active Immunity

When a host is exposed to antigens — which may be living or dead microbes or other proteins — the body responds by making its own antibodies. This is active immunity: the immune system does the work itself.

Active immunity can arise in two ways. It develops through natural infection, when a pathogen gets into the body on its own, or through vaccination, when microbes are deliberately introduced during immunisation. The trade-off is worth remembering: active immunity is slow to develop and takes time to give its full effect, but the protection it provides is long-lasting.

Borrowed Defence: Passive Immunity

Sometimes there is no time to wait for the body to build its own antibodies. In passive immunity, ready-made antibodies are given directly to protect the body against foreign agents. This works fast, but the protection is short-lived because the body has not made — and does not remember how to make — those antibodies itself.

There are some classic examples. A newborn receives colostrum — the yellowish first milk secreted in the early days of lactation, which is rich in IgA antibodies. Even before birth, the foetus receives antibodies from the mother across the placenta. And in emergencies we give preformed antibodies directly, as with tetanus antitoxin or snake antivenom.

How Vaccination Works

The whole principle of vaccination and immunisation rests on the immune system's property of memory. In vaccination, a preparation of antigenic proteins of a pathogen, or an inactivated or weakened pathogen — the vaccine — is introduced into the body.

The body then makes antibodies against these antigens. Just as importantly, the vaccine generates memory B-cells and T-cells that recognise the pathogen quickly on any later exposure. So during an actual infection the immune system responds fast and overwhelms the invader with a massive production of antibodies, and the disease never takes hold.

Passive Immunisation and Safer Vaccines

If a person is infected by a deadly microbe and a quick response is needed — as in tetanus — there is no time to wait for the body to build immunity. In such cases we directly inject preformed antibodies, or an antitoxin (a preparation of antibodies against the toxin). The injection given after a snakebite works the same way, containing preformed antibodies against the venom. This is called passive immunisation.

Biology has also made vaccines safer. Using recombinant DNA technology, the antigenic proteins of a pathogen can be produced in bacteria or yeast, allowing large-scale production and greater availability. The hepatitis B vaccine produced from yeast is a well-known example.

Quick Recap

  • Active immunity — the host, exposed to antigens by natural infection or vaccination, makes its own antibodies; slow to develop but long-lasting.
  • Passive immunity — ready-made antibodies are given directly; fast-acting but short-lived.
  • Examples of passive immunity: colostrum (rich in IgA) for the newborn, antibodies crossing the placenta to the foetus, tetanus antitoxin and snake antivenom.
  • Vaccination relies on immune memory: a vaccine (antigenic proteins or inactivated/weakened pathogen) triggers antibodies plus memory B- and T-cells for a fast future response.
  • Passive immunisation = injecting preformed antibodies/antitoxin (tetanus, snakebite).
  • Recombinant DNA technology yields safer vaccines, e.g. hepatitis B vaccine from yeast.

Solved Examples — Section 7

Q1. Distinguish active from passive immunity in terms of speed and duration.

Answer: Active immunity, where the host makes its own antibodies, is slow to develop but long-lasting. Passive immunity, where ready-made antibodies are given, is fast-acting but short-lived.

Q2. Give the two ways active immunity can be induced.

Answer: Through natural infection (the pathogen enters on its own) and through vaccination/immunisation (microbes or antigens are deliberately introduced).

Q3. Why is colostrum important for a newborn, and which antibody makes it so?

Answer: Colostrum, the yellowish first milk, is rich in IgA antibodies that protect the infant. Since the antibodies are ready-made, this is an example of passive immunity.

Q4. On what property of the immune system is vaccination based?

Answer: On memory. A vaccine makes the body produce antibodies and memory B- and T-cells that recognise the pathogen quickly on later exposure.

Q5. Why is a tetanus patient or a snakebite victim given preformed antibodies rather than a vaccine?

Answer: Because a quick response is needed and there is no time for the body to build its own immunity. Injecting preformed antibodies/antitoxin gives immediate (passive) protection.

Q6. How has recombinant DNA technology improved vaccines? Give an example.

Answer: It allows antigenic proteins of a pathogen to be produced in bacteria or yeast, enabling safer, large-scale production. The hepatitis B vaccine produced from yeast is an example.