Vaccine Immunity. By Dr Ananya Mandal, MD. The immune system of the body functions to protect it against invasion by germs and microbes. When an individual is vaccinated against a disease or an infection, his or her immune system is prepared to fight the infection.Once vaccinated, when the person is exposed to the bacterium that causes it, the body gears up to fight off the infection.This is a mild episode that often goes unnoticed by the sufferer.he work of B cells is called the humoral immune response, or simply the antibody response. The B cell forms memory cells that remember the infection for life and prepares the body for arresting the infection next time around.