THEORIES OF IMMUNITY. 



suppose that the radical combines at once with 

 something else — which I have represented by 

 X — so that the unfortunate conclusion may not 

 be forced upon us, that a skilled chemist has put 

 forth the supposition of the existence of some- 

 thing in violation of the laws of all known 

 chemistry. 



In considering difficulties like these one is re- 

 minded of the remark of Metchnikoff, — spoken 

 of earlier, — that he does not seek to penetrate 

 the laws governing intra-cellular digestion, which 

 has to do with processes as yet absolutely un- 

 familiar to us. 



A way out of the difficulties presented has 

 been found by the suggestion that there is an 

 essential difference between the two forms of 

 immunity — active and passive; that the first 

 (active) has to do with the cell, and should be 

 called Isopathic, and that the second has to do 

 with the fluids (principally the blood) and is the 

 ordinary antitoxic immunity. 



It was said in beginning to speak of the sub- 

 ject of Immunity in the case of those diseases in 

 which the results are due to the production of 



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