Your immune system produces antibodies to fight disease. However, more than this, remembers any pathogens that it has fought off before. In this way it can respond more quickly the next time. This forms the basis of vaccinations, which prepare the body to fight some diseases without actually infecting them. However, some disease, like tetanus, may require extra halp using passive immunisation, if someone is infected.

When your body is first infected with a pathogen, the main task of the immune system is to combat the infection. However, it also produces lymphocytes that remain in your body for many years and remember the pathogen in case it returns. This is called your immune memory and is the reason why you can only catch infections like Chicken Pox once. If you get infected a second time, your immune system is already prepared for the pathogen and can quickly make enough antibodies to kill the infection before any symptoms are felt. Vaccination takes advantage of this feature of the immune system.

Picture 9 shows how the levels of antibodies in the blood build up slowly when you are first exposed to a particular pathogen. It takes over 5 days for the antibodies to reach a level that will fight off the infection. During this time, you will feel the symptoms of the infection and damage will be caused to your body tissues. In serious diseases, this can be fatal.

After you have recovered, your immune system retains a number of memory lymphocytes. These can react quickly if you are exposed to the same antigen in the future. If the same pathogen infects you again, more antibodies are produced in a shorter amount of time. The antibodies quickly reach the level that will protect you and when this happens, you may not even feel any symptoms of the illness. This is called the memory response.

Developed by Edward Jenner in 1796, vaccination has made a major contribution to the fight against infectious diseases. It prepares your body's immune system to prevent infections from diseases such as measles, mumps, rubella, polio and tetanus.

Tetanus is caused by a bacterium (Clostridium tetani) that lives in soil and enters the body through wounds or cuts. It produces a toxin that attaches to nerve-endings in the muscles and causes uncontrollable contractions. It can be fatal because the toxin paralyses the muscles responsible for breathing.

In the United Kingdom, a person is usually vaccinated against tetanus during their regular immunisation programme. Pre-school children receive three doses of the vaccine, which are then followed up with booster vaccinations when they are in their primary and secondary schools. If they get a cut that is likely to contain the tetanus bacterium, a booster injection is only given if the last vaccination was more than 10 years earlier.

If the risk of infection is especially high, an injection containing antibodies against the toxin can be given. This is a type of passive immunity.

Passive immunity is when a person is given antibodies, rather than their own immune system producing them.

One type of passive immunity is used to protect against tetanus. If people have not been vaccinated against tetanus or if they get a cut in a high risk situation (for example handling horse manure), then the toxin could cause them to die before their lymphocytes were able to produce enough antibodies against the toxin. If this is a possibility, the person will be given an injection containing a solution of the antibodies as well as having the wound treated. The antibodies in the injection do not give the person long-term immunity but they do destroy any toxins that are produced while the infection is being treated with antibiotics. For more long-term protection, they will need a series of tetanus vaccinations.

Passive immunity is also seen in the first few months of life. New born babies have a poorly developed immune system. They receive antibodies from their mother through the placenta and in breast milk. This helps them to fight off infections until their own immune system is fully working.