Vaccines work by training the immune system to recognize and combat pathogens, either viruses or bacteria. In order to do this, certain molecules from the pathogen need to be introduced into the body to trigger an immune response. These molecules are called antigens, and they’re present on all viruses and bacteria. Memory cells are then formed to remember the same pathogen for faster antibody production in future infections. Vaccinations increase the level of circulating antibodies against a certain antigen. Antibodies are produced by a type of white blood cell (lymphocyte) called B cells.