How does vaccination work?

Vaccines contain weakened (or attenuated) antigens which cause immunity to a disease without actually giving the person the disease. They are usually adminstered by injection or sometimes orally (by mouth), like the polio vaccine. The antigens in the vaccine cause a type of white blood cell, called a B-cell, to produce specific antibodies which can fight the disease. Sometimes multiple vaccinations are needed to ensure immunity is achieved. The first vaccintion will cause a small amount of antibody production and the production of some memory T cells, this is known as the primary response. This is a slower response because the body has not encountered the antigen before and so must "try out" lots of different antibodies until it finds the specific one to fight the antigen. The second vaccination, will cause a much faster reaction with a greater production of antibodies due to the existence of memory cells from the first vaccination. Memory cells persist in the body so if the individual re-encounters the disease they have immunity.