CHAPTER XIII 



IMMUNITY, GENERAL DISCUSSION 



Immunity. Immunity is a term used to express relative resist- 

 ance to disease. It is denned by Ricketts as follows: " By immun- 

 ity we understand that condition in which an individual or a species 

 of animals exhibits unusual or complete resistance to an infection 

 for which other individuals or other species show a greater or less 

 degree of susceptibility." The converse of immunity is suscepti- 

 bility, or lack of resistance. 



Resistance by the body to infection by microorganisms is 

 due to a considerable number of factors. These may be grouped 

 into two classes the external resistance, due to body coverings 

 and protective devices, and internal resistance, due to tissue and 

 body humor reactions. 



External resistance to infection is of the greatest practical 

 importance. The skin covering the surface of the body is an excel- 

 lent and effective barrier against bacterial invasion. The skin 

 is constantly sloughing off at the surface and being replaced from 

 below. Entrance to the tissues is sometimes effected by micro- 

 organisms through the hair-follicles and the sweat-glands; this is, 

 however, exceptional. The subcutaneous tissues likewise obstruct 

 the inward growth of organisms that have penetrated the skin. 

 The mucous membranes constantly secrete mucus, which is as con- 

 stantly removed, and the bacteria which have been caught go with 

 it. The membranes which line the air-passages catch upon their 

 moist surfaces practically all the bacteria that enter with the 

 inspired air, and few ever reach the ultimate ramifications of the 

 bronchioles, much less the alveoli. The gastric juice of the stomach 

 is markedly germicidal. Many, though not all, bacteria which 

 enter the body with the food are destroyed there. The intestinal 

 juices, particularly the bile, are mildly antiseptic and inhibit the 

 growth of many forms, although they are without effect on others, 

 among them the so-called normal intestinal bacteria. 



121 



