=Vaccination= Vaccination involves injecting a small amount of an inactive form of a pathogen (a disease causing micro-organism) into the body. Vaccines may contain: *A live but weakened form of the pathogen *An inactive form of pathogen (e.g. pathogen that has been killed by radiation) *Fragments of the pathogen *A deactivated form of the toxin produced by a pathogen *A closely related strain that doesn't cause any disease Injecting the inactive form of the pathogen stimulates the immune system without causing any disease. ==Immune memory:== [image:] When we are first infected with a pathogen the a range of non-specific immune defences including, chemical and cellular responses try to clear the infection. Once the pathogen has been processed and the immune system has confirmed that it is a foreign invader we begin to produce antibodies, small proteins that can stick to the virus, deactivate it and tag it for destruction by white blood cells. Antibodies are specific to a particular antigen –some part of the pathogen that can be recognized as foreign (usually a surface protein). It takes some time before antibodies can produced as the immune system has to check that the pathogen is in fact a foreign invader (and not a part of the body itself). By this time, symptoms have usually already developed. When we encounter the same pathogen for the second time specific antibodies can be made much more quickly and can often clear the infection before it gets established and any significant damage is done. A vaccine provides a way of stimulating the immune system without causing the disease (the development of symptoms). It essentially simulates the primary infection so that immune memory is established.