complement system
The initial exposure of the body to an infectious agent e.g. through a vaccine, triggers an immune response most of which rapidly diminishes after the vaccination. However some cells of the immune system called lymphocytes remain with a memory of that pathogen.As a result, if the same pathogen infects a vaccinated person, the memory cells remember and recognize it and rapidly spring into action triggering a much more rapid response specific to that pathogen than the initial exposure, effectively dealing and neutralizing the pathogen.