THE THEORY OF IMMUNITY 139 



body. The catalyst may in that case be present in abund- 

 ance ; but it has nothing to work with, and the substrate 

 flourishes on tissues which would not be attacked if the 

 normal complement were present. In some cases cata- 

 lytic actions may themselves prove harmful. It is perfectly 

 possible that the obscure and dangerous phenomena of 

 anaphylaxis are due to a sudden action of a reversible 

 catalyst breaking down what was before built up, and 

 again setting toxins free. That the immunizing reactions 

 are permanently reversible if a disassociation factor such 

 as acid (Morgenroth and Ascher) or alkali (Sachs) is intro- 

 duced, now seems certain. These workers are quoted 

 by Browning (Brit. Med. Journ., Dec. 6, 1915). 



Such conclusions certainly reinforce the suggestion that 

 in all infections an effort should be made to determine 

 with exactitude the natural combining body used in the 

 body's defence, and that this should be supplied in abund- 

 ance by feeding or injection. On the views expressed the 

 lipoids or other combining bodies found normally in the 

 organism would then be spared, since it is only reasonable 

 that a free lipoid, etc., would be dealt with rather than 

 a fixed lipoid, say in the cell envelope. While vaccines 

 may provoke the catalyst, they cannot always provoke the 

 " complement," although when we consider some drugs 

 (and tolerance of drugs, as has been said, no doubt comes 

 under the head of immunity) they may do so indirectly. 

 Though arsenic when used as a drug is often useful, its 

 value may not always depend on its bactericidal qualities, 

 but on its encouragement of lipoid manufacture. 



Thus finally we see that nutrition itself is but a case 

 of immunity, and, instead of immunity being infinitely 

 complex, on a general view it is no more than an example 

 of the fact that living protoplasm develops machinery 



