Vaccinations:How do they work?

Hi,
I am starting a series (hopefully!) dealing with common vaccination related questions. Hope that they will be useful, comments and questions are appreciated!
How do vaccinations work?
The main purpose of our Immune system is to fight infections. Every day, the body is bombarded with bacteria, viruses and other germs. When a person is infected with a disease-causing germ, the immune system mounts a defense against it. In the process, the body produces substances known as antibodies against that specific germ. The antibodies eliminate the germ from the body. The next time the person encounters the germ, the circulating antibodies quickly recognize it and eliminate it before signs of disease develop.
This is why a child who has had chickenpox will only rarely develop the disease again. The immune system has memory. The next time the child encounters the virus that causes chickenpox, the antibodies destroy the virus before disease causes sickness. Medical experts estimate that the immune system can recognize and effectively combat hundreds of thousands, if not millions, of different organisms, or more.
A vaccine works in a similar way. However, instead of one natural infection, for immunity to develop after a vaccine it usually takes several doses over several months or years. The vaccine contains an inactivated (killed), weakened form of the germ, or a germ component. When introduced into the body, the dead or harmless germ causes an immune response without causing the disease. The immune system develops antibodies that will effectively kill or neutralize the germ if exposed to it in the future. The antibodies circulate in the bloodstream. Vaccination protects a child against infection with a germ without the child ever suffering through the disease.It does this by creating antibodies that fight foreign proteins (antigens) that cause disease.
There are two types of vaccines: live attenuated (weakened) and inactivated forms.
Live attenuated vaccines are produced by ‘weakening’ a bacteria or virus so that it can replicate and produce immunity without causing disease. Examples of live vaccines would include Measles, Mumps, Rubella (MMR), Chicken pox, Yellow fever and Polio drops.
Inactivated vaccines are made of bacteria or viruses that have been modified or killed with heat or chemicals. Repeat administration of booster doses are often needed to maintain immunity with inactivated vaccines. Inactivated vaccines would include Diphtheria, Pertussis, Tetanus (DPT), Hepatitis A, Hepatitis B, Hib, Typhoid, Rabies, Influenza, Pneumococcal, Meningococcal etc.
When these vaccines are given, the immune system starts producing antibodies against the respective diseases. Thus when an actual infection occurs, our immune system ‘remembers’ the previous dose of antigen (given by way of vaccination) and starts fighting the disease immediately, protecting us from the disease!
More information on how vaccines work can be got by clicking on these links;
UNICEF vaccine & National Network for Immunization information