The main purpose of a vaccine is to stimulate the body's immune system to recognize and combat specific pathogens, such as viruses or bacteria. Vaccines contain antigens, which are weakened or inactive forms of the disease-causing organism or pieces of it. When administered, vaccines prepare the immune system to respond rapidly and effectively upon future exposure to the disease. This process helps to prevent illnesses, reduce the severity of infections, and can lead to herd immunity, protecting even those who are unvaccinated.