While immunity is being acquired a person develops symptoms of the disease. But some infections can be prevented b allowing the body to acquire active immunity artificially. This is achieved by the introduction into the body of a vaccine made from dead or weakened micro organisms that are no longer capable of causing the disease itself. When injected into the skin or blood stream9 Or, as in the case of polio vaccine, swallowed) they stimulate B-lymphocytes to produce antibodies but no significant disease symptoms. And if, or when, the body encounters the real disease causing micro organism, it is already protected against infection because the memory cells immediately produce antibodies. Booster shots may be needed at later dates to ensure the active immunity is still effective. Vaccines are available against many formerly dangerous diseases including polio, measles and diphtheria. Immunization programmes for children have virtually eliminated most common childhood diseases, and confer immunity into adult life. But vaccination is advisable for travelers to countries where they may contract potentially fatal diseases not previously encountered.
Sometimes antibodies themselves, from external sources, are injected into the body to provide protection without stimulating the immune system. This is called artificial passive immunity and is useful in providing instant protection where a disease might kill the person before their active immune system has time to come into operation. Examples include gamma globulin, given against hepatitis A infection; am antitoxin to treat tetanus; and anti rabies injection.
