How do Vaccines Work?


Vaccination relies on the ability of the immune system to remember past infections and store products from that immune response in case of a future infection by the same disease. Vaccination copies in a more directed and controlled way, the process of infection and immune response triggered by a foreign invader.

When you are vaccinated against a particular disease, you are being injected with all or part of that disease-causing organism. Your body mounts an immune reaction, producing specific antibodies. These then remain in your system to be activated in the event of a future attack.

Vaccine induced immunity works in four ways:

  1. Antibodies in the gut and airways (the most likely sites of infection) bind onto the disease-causing bacteria or viruses and prevent them from latching on to and penetrating the cells lining these passages.
  2. Antibodies in the airways, gut and tissues bind with and neutralise the poisons released by some disease-causing bacteria. This happens with the diphtheria toxin in the throat and tetanus toxin in the body tissues.
  3. Immune cells called cytotoxic T-cells, kill virus-infected cells. The measles vaccine stimulates this response.
  4. Another kind of T-cell helps other body cells to kill bacteria that have got inside them.

They do this by releasing messenger molecules which act on the infected cells and cause inflammation. The BCG vaccine against tuberculosis does this.


Immunization is described as active or passive. In active immunization, part or all of the disease-causing organism which has been modified in some way is inserted into your body. This causes the body to set up an immune response, produce cells and antibodies to kill the disease and store protective cells in case of future infection. This uses:


1. Non-harmful organisms closely related to the disease-causing bacterium or virus, for example smallpox and cowpox.

2. The disease-causing bacterium or virus itself that has its disease-inducing properties removed. These are known as attenuated vaccines. This form of vaccine is used against measles, mumps, rubella and tuberculosis.


  1. The toxins (poisons) produced by disease-causing bacteria, modified to make them harmless. The current vaccines against diphtheria and tetanus use this method.
  2. The dead disease-causing agent: Whooping cough, rabies and anthrax vaccines are examples of these.
  3. Genetically engineered or purified molecules from the disease causing organism. These are used against pneumonococal bacteria. In passive immunisation, the antibody products of an immune response harvested either from a person who is recovering from the disease or made specifically for use in immunization, are injected into the body. 

In passive immunization, the antibody products of an immune response harvested either from a person who is recovering from the disease or made specifically for use in immunization, are injected into the body.

The BCG vaccine against Tuberculosis.

At the beginning of the 20th century over a period of ten years, two French scientists, Albert Calmette and Camille Guérin, cultured the bacterium that causes tuberculosis in cattle. During this time, the culture acquired stable changes that altered the bacterium to a new species called Bacillus Calmette Guérin (BCG). The changes resulted in a much less virulent kind of tuberculosis which, when used as a vaccine, protects you against the deadly tuberculosis infection.


This depends on the type of vaccine. With inactivated vaccines, you need several booster vaccinations to build up a high state of immunity. With live vaccines the effect is more long-lasting, and one shot in infancy and a booster in old age is generally enough.

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