Immunisation

Different dangerous diseases can be avoided through immunisation - the artificial creation of immunity against infection.

Immunity is the ability of the human body to protect itself from infectious disease. 

Active immunity: 

• Protection that is produced by an individual's own immune system and is usually long-lasting. Such immunity generally involves cellular responses, serum antibodies or a combination acting against one or more antigens on the infecting organism.
• Acquired by natural disease or by vaccination.

Passive immunity:

• Protection provided from the transfer of antibodies from immune individuals, most commonly  across the placenta or less often from the transfusion of blood or blood products including immunoglobuin.
• Protection is temporary - commonly for only a few weeks.

Vaccines produce their protective effect by inducing active immunity and providing immunological memory. Immunological memory enables the immune system to recognise and respond rapidly to expose to natuarl infection at a later date and thus to prevent or modify the disease. 

Antibodies can be detected in blood or serum, but, even in the absence of detectable antibodies, immunological memory may still be present.

From birth and in early childhood, humand are eposed to countless numbers of foreign antigens and infectious agents in the everyday environment. Responding to these stimuli helps the immune system to develop and mature. Compared with exposeure in the natural environment, vaccines provide specific stimulation to a small number of antigens. Responding to these specific antigens uses only a tiny proportion of the capacity of an infant's immune system. If an infant's immune system could be exhausted by multiple vaccines, one would expect vaccinated children to be at a higher risk of serious infections. Studies to investigate whether vaccines increase suspectability to serious infections have shown no evidence of such an effect, with infection rates generally being lower in vaccinated children.

Vaccinated individuals are less likely to be a source of infectionto others. This reduces the risk of unvaccinated individuals being exposed to infection. This means that individuals who cannot be vaccinated will still benefit from the routine vaccination programme. This concept is called population (or herd) immunity.

When coverage is high enough to induce high levels of population immunity, infections may even be eliminated from the country, eg. diptheria. But if high vaccination coverage were not maintained, it would be possible for the disease to return. Vaccination against smallpox enabled the infection to be declred eradicated from the world in 1980. The World Health Organisation is currently working towards the global eradication of poliomyelitis.