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Class 12 Biology Notes - Vaccines A vaccine  is an inactivated form of bacteria or virus that is injected into the body to simulate an actual infection. Since the injected microorganisms are ‘dead,’ they don’t cause a person to become sick. Instead, vaccines stimulate an immune response by the body that will fight off that type of illness. Vaccines can be prophylactic/preventive (example – to prevent or ameliorate/amend the effects of a future infection by a natural or “wild” pathogen), or therapeutic (that treats the disease for example – vaccines against cancer are being investigated). How the Immune System Works? To better understand vaccines, we need to know more about how our body’s immune system works. There are special cells in our bloodstream called  white blood cells . They have the very important job of fighting off foreign invaders such as viruses and bacteria. These invaders are known as  antigens . White blood cells are like the armed forces of o...