A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. Vaccines are made from weakened or killed forms of the microorganism that causes the disease and/or components of the pathogen, such as its toxins or surface proteins. A vaccine stimulates the immune system to recognize the microorganism and create memory cells that can quickly respond to a future encounter with the same pathogen and destroy it before it can make you sick.
When scientists identify a natural or manufactured antigen that will trigger the immune system to create defenses against a disease, it takes about two to four years to produce a vaccine candidate. Government, academic and industry researchers test these candidates in laboratory animals to make sure they work before bringing them into clinical trials with human volunteers.
Once the vaccine is injected, the antigen enters the body through small channels and travels to organs called lymph nodes, where it’s broken down into components that the immune system can absorb. Antigens are then passed to killer T-cells that can target and kill cells containing the pathogen. Plasma B-cells can also bind to the antigen and form antibodies that attack and destroy the pathogen. Some vaccines contain a weakened (attenuated) form of the virus or bacterium itself, while others — such as live-attenuated rabies and measles vaccines and inactivated polio and tetanus vaccines — use only specific parts of the germs, such as their proteins or sugar casings.