ON IMMUNITY TO BACTERIA 345 



largest quantity, and when the number of molecules reaches a 

 certain amount they clump, whilst those with less affinity have 

 not yet absorbed sufficient. Probably exactly similar phenomena 

 occur in the opsonization of bacteria. Some organisms are avid 

 for opsonin ; others take it up with difficulty, and a large concen- 

 tration of the substance is necessary before they are prepared for 

 phagocytosis. This probably explains the fact that the number of 

 bacteria taken up by the leucocytes does not increase pari passu 

 with the amount of opsonin present. 



The bacteria which remain immune to the defensive mechanisms 

 will be enabled to live, and in their descendants the conditions for. 

 natural selection will occur variations and an adverse environ- 

 ment. The weaker forms i.e., those which, by the absence of a 

 defensive layer or .the presence of numerous receptors on which 

 the opsonins or bacteriolysins of the host can seize, or those 

 which do not elaborate a powerful toxin will be destroyed, and 

 the more virulent forms will survive, so that a gradual selection 

 of the latter will ensue, and the invaders rapidly increase in 

 virulence. And it must not be forgotten that bacteria multiply 

 with great rapidity, division sometimes occurring in a quarter of 

 an hour, so that nearly a hundred generations are passed through 

 in a day, and abundant opportunity for evolutionary selection 

 occurs. From what we know of the virulence of typhoid bacilli 

 and of pneumococci when causing disease and when living 

 parasitically without the body, there is reason to believe that this 

 increase of virulence always takes place in infections. 



Here, then, the standing forces of the body are insufficient to 

 deal with the invader, and the latter increases in virulence and 

 numbers during the early stages of the struggle. This process 

 will continue until the latent reserve forces are mobilized and 

 fresh defensive substances brought into action. These are 



(1) antitoxins, whether to exotoxins or endotoxins, or the toxins 

 may be dealt with in one or other of the methods discussed under 

 Antitoxic Immunity ; but in any case it is probable that the develop- 

 ment of some degree of toxic immunity, especially, perhaps, of 

 the leucocytes, must precede the destruction of the bacteria. 



(2) Increased amounts of alexin-opsonin having a specific action 

 on the organism in question. We have already noted some of 

 the gaps in our knowledge of this subject, and shall, perhaps, 

 again have occasion to refer to the difficulties in explaining 

 its action, but there can be little doubt of its importance in 



