IMMUNITY (from Lat. jinni unites, exemp tion, from immunis, exempt, from in, not mi nis, serving, from munus, duty). Resistance of the living organism to infection. There is an astonishing difference in the resistance shown to the invasion by germs of disease by certain ani mals of allied nature. Human beings. cattle, and guinea-pigs show great susceptibility to tuber culosis; while the cat, dog, and horse resist it. Man easily becomes a victim of typhoid fever, while domestic animals never suffer from it. Im munity is either (1) natural or (2) acquired. Natural immunity is a congenital insensitiveness to infection or contagion which is possessed by certain individuals or families, or which exists as a racial characteristic. The same individual may be immune against a certain disease at one age and become susceptible to it at another age; thus infants are almost entirely immune against yellow fever, and but few cases occur among them. The Arab is said to be absolutely im mune to typhoid fever. The Japanese are im mune to scarlet fever. Acquired immunity is a condition of insusceptibility which results from passing through an attack of the disease, or from being inoculated with the poison of the disease, which results in a change in the blood. Imniu• nity to yellow fever results after one attack of the disease. One attack of typhoid fever is almost a certain guard against another, a statement which is true also of scarlet fever and measles. Exper
imental immunity is acquired immunity result ing from introducing, for experimental purposes, attenuated micro6rganisms into susceptible ani mals. and thereby causing the production of im nmnity to virulent forms of those organisms. The house mouse and the white mouse, naturally very susceptible to anthrax, are rendered immune to this disease by receiving an inoculation of blood from a convalescent tetanic animal.
Immunity is lost or destroyed in a number of ways. It may be destroyed by (1) variation from the normal temperature. Pasteur found that chickens. naturally immune to anthrax, became susceptible to the disease after being plunged into a cold bath. (2) Altering time chem ical composition' of the blood by ehatiging the diet. or by injection of drugs, (3) Loss of strength. or exhaustion, is a cause of loss of immunity, as Roget demonstrated by inoculating previously immune white rats with anthrax, after compelling them to work at revolving a wheel. (4) Remora/ of the spleen is followed by susceptibility to disease (Bardach). (5) Combining various microorganisms destroys im munity in smile experimental eases.