Pathogens are microbes that cause diseases. Vaccines allow a dead or altered form of the disease causing pathogen to be introduced into the body, which contain a specific antigen. This causes the immune system, specifically the white blood cells, to produce complementary antibodies, which target and attach to the antigen. When a white blood cell engulfs and digests a pathogen it is called phagocytosis.
During the primary infection the antibodies slowly increase, peak at around ten days and then gradually decrease. A second exposure to the same pathogen causes the white blood cells to respond quickly in order to produce lots of the relevant antibodies, which prevents infection.