THE CHEMICAL MECHANISMS OF DEFENCE 1087 



taken up by the leucocytes after treatment with the serum of the individual, 

 and then comparing it with the number taken up by the same leucocytes when 

 the bacteria have been treated with the serum of an average individual. 



We thus see that immunity, whether innate or acquired, is extremely 

 complex in character and may depend on one or more of many factors. 

 The immunity of an animal to any given infection may be determined by 

 the absence of receptor groups in his body for the toxin excreted by the 

 microbe responsible for the infection, or by the fact that the receptor groups 

 are present but are confined to tissues on which the toxophore group can 

 have no influence. Thus e.g. an attachment of the tetanus toxin to a 

 connective tissue cell would be without effect on the health of the body. 

 Again, immunity may be due to the efficacy of the phagocytes, either of the 

 fluids or the connective tissues, in ingesting and destroying the micro- 

 organisms, and this, as we have seen, may again be dependent on the presence 

 or absence in the body fluids of substances which, while not destroying the 

 micro-organisms, render them more accessible to the action of the phago- 

 cytes. In those cases where the infecting organism secretes a specific toxin, 

 the main fine of defence and the main factor in the production of immunity 

 is the formation of specific antitoxins to the poison in question. Finally 

 there may be produced as a result of the excess of micro-organisms substances 

 such as the amboceptors, which render the micro-organisms susceptible to 

 destruction by the complements or cytases normally present in the circu- 

 lating fluids and possibly themselves derived from the activity or destruction 

 of the leucocytes and other phagocytes of the body. 



In this short description we have been able to touch only upon the most 

 salient features of the immunity problem. The question enters strictly into 

 physiology since, as we have seen, it involves adaptations on the part of 

 the organism to change in itself or its environment. For the practical 

 application of these facts, as well as the consideration of the minuter details 

 and exceptions, we must refer the student to works especially dealing with 

 the subjects of infectious diseases and immunity. 



