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Artificial passive immunity

When antibodies made in one individual are extracted and injected into another individual to protect them against a disease such as tetanus or rabies.

Artificial active immunity

The process by which the body produces antibodies to an antigen that it has been exposed to through immunisation.

Natural passive immunity

The process by which a mammalian mother passes antibodies to her baby through the placenta and in breast milk.

Herd immunity

If a high percentage of a population is immune to a disease the disease cannot be passed on because it cannot find new hosts.

Immunisation

The process of exposing individuals to inactivated antigens to allow them to develop active artificial immunity.

Vaccination

A small amount of dead or weakened pathogen is introduced into the body. It prepares the immune system to prevent future infections with the live pathogen.

Antiseptic

A chemical which can destroy microorganisms. Antiseptics are applied to the surface of the skin or to living tissue to reduce the possibility of infection.

Smallpox

A killer disease until it was eradicated by 1980. Infected individuals are covered in skin sores and damage to body organs can cause death.

Polio

Viral disease causing paralysis which has been eradicated from most countries in the world by a vaccination programme

Preventing the spread of disease

Practical measures

Understanding the causes of communicable diseases makes it easier to prevent them spreading. This is an important part of reducing the burden of disease on the human population relatively cheaply and very effectively. The key thing is to prevent the transmission of pathogens from one host to another. This can be achieved in a number of ways including:

  • Simple hygiene measures in the kitchen such as hand washing before preparing food, using disinfectants to keep surfaces clean and storing raw food and cooked food separately
  • Always washing hands after using the toilet
  • Regular hand washing and use of alcohol antiseptic gels in hospitals and care homes by health workers, care assistants and visitors
    Antiseptic hand gel

    Using the antiseptic hand gels provided by hospitals has been shown to reduce infections – but many people still walk past and ignore it (Photo credit: Tobia Arhelger)

  • Keeping vectors of disease at bay, whether this is keeping flies out of the kitchen or using protective meshes and insecticide-impregnated mosquito nets to prevent mosquitoes biting and spreading diseases such as malaria
    Mosquito net

    Keeping people and mosquitoes apart using simple and relatively cheap measures can have a major impact on the spread of disease. (Photo credit: Wellcome Trust/A Stich)

  • Educating people to raise their awareness of the way diseases can be spread
  • Sewage treatment systems or well-sited pit latrines to reduce faecal contamination of drinking water

Natural immunity

When a pathogen gets into your body you become ill because it takes time for your primary immune response to work. That is when you experience the symptoms of disease. However, the second time you encounter a pathogen, your immunological memory comes into play. Your secondary immune response, based on the T and B memory cells left in your blood after the first exposure, results in the pathogens being destroyed before they can cause the symptoms of disease.

Primary and secondary responses of the body to infection

The primary and secondary responses of the body to infection

One useful evolutionary adaptation is that female mammals pass on a level of immunity to their offspring. This is called natural passive immunity. Antibodies to some diseases are passed from the mother to her fetus through the placenta. After birth, the newborn baby gets extra protection from antibodies passed through the mother’s milk. They provide protection against infection until the baby’s immune system is fully active and has had a chance to meet a range of different antigens, increasing the chance of survival of the offspring.

Breast feeding infant

Young mammals don’t just get food from their mother’s in the milk – they get antibodies that help to protect them from disease as well. (Photo credit: US department of agriculture/Ken Hammond )

Induced immunity

One effective way to prevent the spread of disease is through induced immunity. Scientists have developed ways to give us immunity to a disease without having to suffer the illness first. Immunisation protects individuals and communities and saves millions of lives every year. It is widely used against diseases that can kill or cause serious damage if you are infected before your immune system can produce the antibodies and T killer cells needed.

Artificial active immunity is the basis of immunisation. Small amounts of the antigen known as the vaccine (often dead or weakened pathogens or fragments of the antigens from the surface of the pathogen) are introduced to the body where they induce a primary immune response, but without the symptoms of disease. Then when you meet the live pathogen, a secondary immune response is triggered which deactivates or destroys the pathogen before it can cause disease.

Immunisation

Immunisation – the development of artificial active immunity

If a disease is to be eradicated or just successfully controlled using immunisation, it is important to reach a level of herd immunity. This occurs when a big enough proportion of a population is vaccinated leaving it difficult or impossible for a pathogen to spread through a community because almost everyone is immune. Herd immunity means not only are individuals being vaccinated protected against a disease, but also vulnerable members of a population are effectively protected. This provides protection for tiny babies, very old people, people who cannot be vaccinated for various reasons and people who are already ill or have a compromised immune system. Smallpox has been completely eliminated through vaccination in this way, by 2015 polio had been eliminated from all but three countries in the world and many other diseases have been reduced or eliminated from countries where the vaccines are used.

Graph of the effect of introducting polio vaccines on levels on polio in US

This graph shows the effect of introducing polio vaccines on levels of polio in the US – it has now been completely eliminated.

An alternative to artificial active immunity is artificial passive immunity. If someone is infected by a dangerous pathogen, antibodies already produced by another individual to that pathogen may be injected. The antibodies may be from a person who has already had the disease or even from a different species. They do not last long or confer long term immunity – but they can prevent infection by potentially lethal pathogens such as tetanus and rabies. One of the treatments trialled during the 2014-5 Ebola epidemic in East Africa was the use of blood or plasma from people who had survived the disease. The idea is to give new patients artificial passive immunity against the potentially deadly virus.