Immunity is the natural defenses that allow one to avoid or fight off disease, infection, or other types of biological invasion of the body. Immunity is generally divided into two major types: innate immunity and adaptive immunity.

Innate immunity is nonspecific, and individuals are born with it. Physical barriers like intact skin, chemical barriers like enzymes in tears and saliva, and events like fever comprise the innate immunity.

Adaptive immunity is a response to immune system challenges. It can be subdivided into two types: passive immunity and active immunity. Passive immunity is gotten through breast milk (which contains antibodies, chemical substances tailored to fight off specific invaders), or from certain immunizations, for example a shot of tetanus antitoxin, which contain the antibody obtained from some other person or form an animal.

Active immunity results when the body makes antibodies to a pathogen because it was actually exposed to that pathogen. This can occur when one gets an illness and subsequently becomes immune, or through administration of dead or weakened pathogens in a vaccination.