A vaccine is a biological preparation that provides acquired immunity against a specific infectious disease. It contains an antigen, which is usually a weakened or killed form of the disease-causing microorganism, one of its surface proteins, or one of its toxins. The antigen in a vaccine causes the immune system to recognize the germ as unwanted and creates defenses, such as antibodies, to fight it. The body can then recognize and destroy any live germs it encounters later.
Vaccines help prevent illness, and can even save lives by stopping disease from spreading widely in the population. This benefits the health of children, families and communities, reduces strain on healthcare systems, and allows people to focus on work, school, care for their loved ones and fulfill their potential.
Vaccines train the immune system to protect against harmful invaders without making you sick. Some use a weakened pathogen, others use inactive viruses or parts of bacteria (like the capsid proteins that make the polio virus grow). Regardless of what they contain, vaccines trigger your immune system’s primary response, and generate memory cells to quickly respond the next time a dangerous invader comes along. These memory cells keep a kind of wanted poster for the invader in their blood and know exactly which tools to call upon to attack it. This is called cell-mediated immunity. Vaccines also use adjuvants, which make the immune response stronger. These may be made of sugars, gelatin, human serum albumin from screened blood donors, or chemical compounds like 2–phenoxyethanol that are safe to use in small amounts and are used as stabilizers to ensure the vaccine ingredients stay mixed together during production and storage.