CHAPTER II 

 THE PROTEOLYTIC MECHANISM IN OPERATION. 



Such, then, is the immunizing mechanism of the body as I 

 conceive it: (1) a system of lymphoid tissues, comprising lym- 

 phatics, bone marrow, and spleen, and sending into the field 

 legions of leucocytes and still greater legions of red corpuscles 

 to support them; (2) body-cells of many types standing in the 

 background, each equipped only for individual defense; and (3) 

 the liver and kidneys as the chief excretory organs of the by- 

 products of the conflict. This is the system which guards the 

 body from within, as skin and mucous membrane guard it from 

 without, against the intrusion of foreign proteins of every type, 

 and against the products of proteid activity. 



Some further details as to manner of working of this impor- 

 tant mechanism may perhaps be presented to best advantage if we 

 make inquiry as to precisely what takes place on the various 

 occasions when the efficacy of the defensive mechanism is put 

 to an exceptional and decisive test. 



Let us assume a case in which a few molecules of unbroken 

 protoplasm have found their way through the intestinal wall 

 evading the pepsin and trypsin and erepsin of the digestive tract 

 and enter the blood stream. This is not a strictly normal oc- 

 currence, to be sure, but it must be a very common one, under 

 slightly maladjusted conditions of digestion, as the experiments 

 already cited show. In these experiments the proteins proved 

 thus to enter the circulation unbroken, including those of egg 

 albumen, blood serum, and milk. So we may fairly assume that 

 any of the ordinary food proteins may on rather frequent occa- 

 sions find their way in small quantities into the blood stream. 

 We have now to inquire what happens to them there. 



The inquiry has practical interest for the clinicism no less than 

 for the physiologist, because it is obvious that it applies equally 

 to the bodily activities responsive to the therapeutic introduc- 

 tion of a foreign protein, through the hypodermic injection of a 

 serum or vaccine or proteal. There are very important quanti- 

 tative differences, however, evidenced in the varying degrees of 

 toxicity, as interpreted by the clinicism. 



Let us first consider the case in which the introducing protein 



comes through the intestinal wall, through partial failure of the 



normal digestive enzymes. Then, according to assumption, the 



protein is a mere fragment of a normal food stuff; but it is a 



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