298 Chapter IX 



cytases are only free in the animal during phagolysis and that under 

 normal conditions the cytases remain closely bound up with the 

 phagocytes. This statement is based upon a large number of well- 

 established experimental facts and must therefore be accepted as 

 proved. It does not, however, affect any fundamental principle of 

 Ehrlich's theory. On the other hand the bases of Ehrlich's theory 

 affect none of the main features of the theory I have developed. The 

 doctrine which regards acquired immunity as a particular case of 

 resorption may be reconciled with the conception of amboceptors. 

 But it accords equally well with Bordet's conception, according to 

 which the fixatives act not as intermediary substances between the 

 micro-organism and the cytase, but as substances which sensitise the 

 micro-organisms for the penetration of the digestive ferment. This 

 delicate question has not yet been definitely settled, but Bordet's 

 experiments described in Chapter IV are greatly in favour of this 

 view. 



Neisser and Wechsberg^ have tried to obtain some idea of the 

 [313] manner in which the fixatives act on the micro-organisms and have 

 recorded a series of very interesting facts. They have shown that 

 these substances only bring about the destruction of bacteria when 

 they are in certain relations with the cytase. Mixtures of fixatives 

 and cytases in which the former are found in excess not only do not 

 kill the micro-organisms but even allow them to develop abundantly. 

 To attain this result Neisser and Wechsberg mixed constant quantities 

 of bacteria and normal serum containing cytase with variable quanti- 

 ties of the serum of immunised animals heated to 56° C. As we 

 know, this specific serum, as the result of being thus heated, is 

 deprived of its cytases, but may be readily made active again by the 

 addition of normal, unheated serum. This paradoxical fact, demon- 

 strated by Neisser and Wechsberg can, in their opinion, be explained 

 only by Ehrlich's theory of amboceptors. When these bodies with 

 double affinities are found in too large quantity as regards the cytase, 

 it may happen that one part only of those which combine with the 

 receptors of the micro-organisms succeed in linking to themselves the 

 molecules of the active ferment. The amboceptor being by itself 

 incapable of destroying the micro-organism, can be injurious to it 

 only on condition that it brings cytase. Consequently as the amount 

 of this cytase is too small for the much larger number of ambo- 

 ceptors we can readily conceive that the micro-organisms may profit 

 1 Munchen, med. Wchnschr.^ 1901, p. 697. 



