Vaccination is the process in which a substance (antigen) is injected, swallowed or otherwise taken into the body in order to trigger a reaction in the immune system. This reaction the production of proteins (immunoglobulins) known as antibodies.
A very small number of the antibodies against a particular antigen remain in the body for many years or even for the rest of the patient’s life. While these antibodies remain, the body retains the memory of how to produce them so that at a later time, if an antigen to which it has been previously exposed enters the body, the body can rapidly produce more antibodies to destroy it. This rapid response enables the body to defend itself against antigens that may be harmful.
All viruses and bacteria have proteins on their surface, which may act as an antigen to trigger an antibody response. This is the basis of vaccination. A harmless part of a virus or all of a killed virus is introduced into a person to trigger an antibody response without causing the disease for which the virus is normally responsible. This then gives long-term immunity against the virus.
The immune system sometimes needs several prompts by repeated doses of the antigen in order to obtain long term immunity. A patient whose immune system has responded appropriately to the vaccination so that they have immunity is said to have seroconverted (become seropositive). This means that the antibodies against the virus can be detected by the appropriate blood test. A small number of patients may not form adequate levels of antibodies in response to an antigen and are described as failing to seroconvert (they are seronegative).
Viruses are far simpler structures than bacteria and as a result, vaccines against viruses are far easier to produce than those against bacteria or even more complex parasites. The majority of vaccines available act against viruses. There are a few against bacteria and none as yet against parasites, although research is continuing on a vaccine that will act against the malaria parasite.
For the same reason, once a person has had a viral infection (eg. chickenpox), they normally have lifelong immunity to that disease due to the antibody response that the first infection triggered in the patient.
Many other substances can act as antigens, including all types of cells, many chemicals, foods, dusts, pollens, etc. In most vases the body recognises that these antigens are not harmful and has no or a minimal reaction to these antigens. In some people the antibody response to a particular antigen may become excessive to create an allergy reaction due to an over reaction by the immune system.