PART III 



CHAPTER VIII 



IMMUNITY. THEORIES OF IMMUNITY 



IN the preceding chapter on the mechanism of infection and the pro- 

 duction of an infectious disease the statement was frequently made that 

 the microparasites of disease are required to overcome the defensive 

 forces of a host which are ever on guard to protect the organism against 

 parasitic invasion and infection. Certain of these defenses are natural 

 to the host, and in a great majority of instances suffice to protect the 

 body against invasion and infection with bacteria, animal parasites, and 

 various inanimate and injurious substances. When, however, these 

 natural defenses are broken down and infection has occurred, the body- 

 cells are not usually rendered powerless and completely overcome, for 

 the products of infection serve as a stimulus to the body-cells, calling 

 forth renewed cellular activity and the production of various specific 

 defensive weapons, termed antibodies, which maintain an incessant 

 struggle against the invading pathogenic agents in an effort to rid the 

 body of them and to neutralize their products. 



Just as microparasites have various offensive weapons, consisting 

 chiefly of their toxins, so, in like manner, the defensive forces of the host 

 are numerous and even more complex. If the toxin of a microorganism 

 is its chief pathogenic weapon, as, e. g., the soluble and extracellular 

 toxin of the diphtheria or the tetanus bacillus, then the body-cells pro- 

 duce an antitoxin as their chief defensive force. If the offensive weapon 

 is largely in the nature of an endotoxin, as, for example, the endotoxins 

 of the typhoid or the cholera bacillus, then a chief antibody is in the 

 nature of a bacteriolysin, which endeavors to dissolve the bacillus in an 

 effort, as it were, to attack the enemy in his stronghold. In other in- 

 fections, especially those due to the pyogenic cocci, certain of the body- 

 cells, and chiefly the polynuclear leukocytes, are observed in the tissues 

 to have engulfed the invaders bodily (phagocytes) in an endeavor to 

 digest them and neutralize their products. In addition to these chief 

 antibodies, there are others that appear to aid them in their work. 



That one attack of many of the infectious diseases may protect the 

 individual against subsequent attacks, or at least render subsequent 

 attacks mild and harmless, is well known. In India and the East for 



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