Immunity is the power of an individual to resist or overcome an infection.  Immunity relates primarily to bacterial and viral diseases, since little or no immunity develops in connection with most fungal, protozoan ,and worm infections.

Phagocytosis and antibodies

The body has been found to react in definite ways to particular pathogens. Of special note are phagocytosis and the production of antibodies. Many kinds of pathogenic bacteria are devoured in great numbers by certain cells called phagocytes. These cells are of two types : motile cells (certain white blood cells) and so called fixed tissue cells. The motile phagocytic blood cells are attracted to bacteria at the site of infection and become an important constituent of pus.

Most infections involve what is called an antigen-antibody reaction. Antibodies are protective substances produced by the body in response to antigenic stimulation. Some antibodies react with toxins to neutralize them. Tetanus antitoxin and diphteria antitoxin, for example, neutralize, respectively, the toxins of tetanus and diphtheria. Other kinds of antibodies are known to enhance phagocytosis.

Acquired immunity

Immunity may be either active or passive. In active immunity the protection against the disease results from the body's own efforts in some type of antigen-antibody reaction. A person may develop active immunity by having the disease or by being inoculated with the appropriate antigen (in the form of a vaccine or toxoid). In immunization against mumps and measles, for exampld, the antigen is the living attenuated (weakened) virus.

In passive immunity protective antibodies are either artificially transmitted to the person by injection or are transferred from mother to offspring via the placenta.

Carriers represent immunity without eradication. Usually after a disease the bodyrids itself of the pathogen. Occasionally, however this does not happen, and a condition of mutual tolerance is set up. The person continues to be a carrier of the infection without showing evidence of disease. The person and the infective organism have acquired the ability to tolerate each other. Carriers of diphtheria, typhoid fever, cholera, meningococcal meningitis, streptococcal disease, malaria, amebic dysentery, and poliomyelitis are known to exist. Many epidemics have been traced to such carriers. In some instances a lowered vitality of the carrier may result in a recurrence of active disease in the individual

Natural immunity

Somewhat over and above the body's acquired immunity is the matter of natural immunity. This type of immunity may be the heritage of a species, race, or individual. Species immunity is one peculiar to a given species. A racial immunity is one possesed by a race.