What is the substance in a vaccine that stimulates the immune system?

Influenza vaccines are made with either a weakened or dead version of the flu virus that you are getting the vaccination to prevent. The annual flu vaccinations in the US contain the three types of flu viruses that are most likely to be causing illness in that particular flu season, as determined by epidemiologists and virologists. The vaccines are called Live Attenuated (which means weakened) vaccines when they have active influenza viruses in them that provide immunity once your body reacts to the presence of the virus. But they are too weakened to actually make you sick. The annual flu vaccines that are administered in a nasal mist are made with LAIV (Live Attenuated Influenza Vaccine). Those flu vaccinations that are given with a needle are made with inactivated "dead" virus particles. When your immune system is alerted to the presence of these viruses, it provides an immune response that produces antibodies to prevent that type of virus from being able to make you sick in the future. See the related questions for more details about the ingredients in flu vaccines and other related information.