84 ELEMENTS OF BIOLOGY 



mammals and birds. In other words, certain bacteria which produce 

 disease in lower forms do not find the human protoplasm a suitable 

 food for their subsistence. Similarly, some lower animals are im- 

 mune to certain diseases that affect Man. In some cases natural 

 IMMUNITY has a somewhat different basis. For example, it is ap- 

 parently not the lack of suitable food material in the body of the 

 horse that renders that animal immune to diphtheria, but the fact 

 that the body cells of the horse provide a superabundance of anti- 

 body that destroys the infecting bacteria and their toxins. Therefore 

 the symptoms of the disease do not develop. 



Acquired Immunity: This is obtained by having had the dis- 

 ease. It may persist for a period of years. Immunity acquired in this 

 way is usually explained as follows: When the toxins of the micro- 

 organisms appear in the blood stream, the body cells, or certain of 

 them, are stimulated by these toxins and respond by producing anti- 

 bodies in their metabolic processes. This is a type of stimulation 

 known as chemical stimulation. If the antibodies are produced in 

 sufficient quantities the progress of the disease is checked. The 

 metabolic process which produces the antibodies once initiated does 

 not immediately cease when the invading organisms have been 

 driven to extinction, but continues to produce the antibodies for a 

 longer or shorter period; in some individuals and in some diseases, 

 throughout life. Thus new infections meet with a supply of anti- 

 body that either totally prevents their development, or results in 

 the production of only a mild form of the disease. Unfortunately 

 for Man, not all diseases have this effect on human body cells and 

 as a result no immunity of any appreciable duration is conferred by 

 having such disease. For example, immunity acquired by having 

 had a common cold is of short duration; attacks seem to succeed 

 each other by re-infection from local cultures in the nose and throat. 

 On the other hand, smallpox confers an immunity that may persist 

 for life. Immunity against one type of pathogenic bacteria affords 

 no protection against other types. 



