IMMUNITY TO TOXINS 137 



affords a sufficient explanation of the differences in susceptibility 

 of the " susceptible " animals which figure in Knorr's list, and that 

 the cells of the central nervous system in the warm-blooded verte- 

 brates differ but little in their relation to tetanus toxin. Thus, in 

 the case of the horse and the guinea-pig little toxin is bound to the 

 body cells, and practically the whole amount makes its way to the 

 brain wherever the injection is made. In the rabbit the body cells 

 can absorb more, so that if only a small dose is given none may 

 reach the more important and susceptible organs. The fowl marks 

 the other end of the scale ; the tissue cells must have an enormous 

 avidity for toxin, and, unless absolutely gigantic doses are given, 

 absorb the whole of it. 



The objection may be raised that, the receptors of the body cells 

 being of the same nature as antitoxin, the resulting combination 

 of body-cell receptor and toxin should undergo dissociation, the 

 poison being gradually passed on to the central nervous system, 

 in the cells of which dissociation does not occur, since poisoning 

 takes place ; this refers to the case, e.g., of the fowl. It is true 

 that such a process appears to occur in vitro. If tetanus toxin be 

 mixed with emulsions of many of the tissues, the two enter into a 

 loose combination, so that if the fragments of toxin-charged tissues, 

 after thorough washing in normal saline solution, are allowed to 

 soak in that fluid, toxin is gradually liberated. It is only in the 

 case of the central nervous system that stable non-dissociable 

 compounds are formed. Several interpretations of the apparent 

 anomaly might be suggested. It is, for instance, very probable 

 that here, as in so many other phenomena in immunity, the 

 real defensive cell is the leucocyte. Thus, Metchnikoff found that 

 if he injected tetanus toxin into the fowl, and then, after an appro- 

 priate interval, excited an aseptic exudate composed largely of 

 leucocytes, the fluid thus obtained would excite tetanus in a sus- 

 ceptible animal. Perhaps the chain of phenomena is as follows : 

 The toxin first unites with the connective and other tissue cells, 

 and so the amount that the leucocytes have to deal with at first is 

 greatly diminished ; dissociation takes place, and there is a steady 

 stream of toxin from the tissues to the blood. This is dealt with 

 by the leucocytes, which thus have time to increase in numbers 

 (and even in the insusceptible fowl the result of the injection is to 

 cause a marked leucocytosis), and perhaps to become immune. It 

 is useless to deny that these phenomena are difficult to harmonize 

 with the side-chain theory as first expounded, or that Metchnikoff 



