A vaccine is an injection, nasal spray or drop of liquid that helps your body make antibodies to fight a specific disease. It takes from several months to multiple decades for laboratory researchers funded by the government, academic foundations or industry to develop and test a vaccine. A single dose of a vaccine contains a very small amount of the active ingredient, which is usually a weakened or inactivated virus, bacteria or piece of a germ. It also contains stabilizers, preservatives, adjuvants or other ingredients to help it work properly and safely.
The immune system uses a complex process to recognize and destroy viruses and bacteria that cause diseases. After a pathogen enters the body, special cells called antigen-presenting cells (APCs) circulate throughout the blood and lymph nodes, looking for foreign invaders. APCs then display a part of the virus or bacteria on their surface, like a flag, so other immune cells can find it. Once other immune cells see the antigen, they release Y-shaped proteins called antibodies that attack and destroy the pathogens.
Once your body makes enough antibodies to eliminate a pathogen, the memory cells it created stay on the lookout for the pathogen and quickly respond if it tries to invade again. This is why vaccination protects you from getting serious or life-threatening diseases such as tetanus, diphtheria, measles, mumps, pertussis (whooping cough) and polio.