IMMUNITY TO TOXINS 133 



able that a very large excess of antitoxin present in the blood may 

 have some slight power of attracting toxin from the tissues when 

 the union is but recent. This, however, is quite unproved, but it 

 is certain that cells which have been acted on by the toxin are not 

 benefited by the subsequent action of the antidote. 



It is hardly necessary to point out that antitoxin is devoid of 

 bactericidal action, and that the removal of the infective agent is 

 brought about by other mechanisms in most cases, perhaps, by 

 phagocytosis. Thus we shall see, in the case of diphtheria, that 

 there is a rapid rise in the opsonic index about the time of the 

 improvement in the local lesions ; and we can hardly doubt, though 

 direct evidence is lacking, that the preservation of the leucocytes 

 from the deleterious action of the toxin when the latter is neutral- 

 ized by means of antitoxin also plays its part in facilitating 

 phagocytosis. 



We now turn to the subject of natural immunity, an exceedingly 

 difficult one, and one that is very far from being understood. What 

 follows is for the most part extremely hypothetical. 



It must be borne in mind that, though we speak of animals as 

 susceptible or as immune to a given toxin, no hard-and-fast line 

 can be drawn between the two conditions, and in some cases an un- 

 broken series can be constructed between the most susceptible and 

 the most resistant species. This is well seen in the case of tetanus. 

 According to Knorr, the horse is the most susceptible animal 

 to the toxin of this disease ; the guinea-pig requires twice as much 

 toxin per kilogramme of body- weight to constitute a lethal dose, 

 the goat 4 times as much, the mouse 13 times, the rabbit 2,000 

 times, and the hen 200,000 times. Von Behring confirms these 

 figures in general, but finds the mouse rather more susceptible 

 than the horse. With the exception of the fowl these animals are 

 all " susceptible." Of the ".insusceptible " animals the hen is sus- 

 ceptible to large doses, but is also affected by much smaller ones 

 if the injection be intra-cerebral. Other animals are still more 

 resistant, such as the tortoise, lizard, cayman, larva of Oryctes, 

 etc. The effect of other toxins on different species of animals 

 has not yet been fully investigated. 



It is necessary to realize that, before we can say that an animal 

 is insusceptible or immune to the action of a given toxin, it is 

 necessary for the animal to be kept, after injection, at a tempera- 

 ture at which this toxin can act. This subject has already been 

 mentioned in connection with the functions of the haptophore and 



