136 NATURAL IMMUNITY TO TOXINS 



the toxophore group only acts at a raised temperature. In the 

 alligator we must assume that the elevation of the temperature 

 has a similar but less marked effect ; it brings the toxophore group 

 into a state of activity in which it is inassimilable by the molecule 

 of cell protoplasm to which it is anchored, although the latter is 

 not injured in any way. The result is, of course, the production 

 of antitoxin, which approximates to the formation of the pre- 

 cipitins which occurs when non-poisonous foreign proteids are 

 injected. 



Thus it would appear that at the last analysis this form of 

 immunity is in reality an expression of cell nutrition. A cell 

 which can nourish itself on a given toxin is naturally immune to 

 its action. It is hardly necessary, however, to say that there is 

 no direct proof that any cell can extract nourishment from a 

 bacterial toxin. 



There is a third, and in some respects more interesting and 

 important, form of immunity to toxins which is readily conceivable 

 on theoretical grounds, and the occurrence of which is capable of 

 experimental proof. We may define it as immunity due to the 

 fact that some of the cells are insusceptible to the action of the 

 toxin, but have a great combining affinity therewith, and thus 

 shield from its action the susceptible cells, in which the combining 

 affinity is less. We have already referred to the most striking 

 and best known example, that of the immunity of the fowl to 

 tetanus toxin injected subcutaneously as compared with its sus- 

 ceptibility when the injection is made direct into the brain. Some 

 authorities have attempted to explain this on the assumption that 

 there^is a barrier en route which prevents the access of the toxin 

 to the central nervous system. But if by this we are to imagine a 

 physical barrier in the shape of a layer of endothelium or other 

 tissue between the blood and the cells of the brain, the explanation 

 is inadequate, since it would leave unexplained the nature of the 

 immunity of this living barrier, nor would it explain the rapid 

 disappearance of the toxin from the blood. The true explanation 

 is certainly that the toxin combines rapidly with the cells of the 

 body, or perhaps, as we shall see later, the leucocytes, and is 

 thereby prevented from gaining access to the central nervous 

 system. These relatively unimportant cells we must imagine to 

 have the same nutritive relations to the molecules of toxin as have 

 the cells of the heated alligator ; the two can unite, but the proto- 

 plasm is neither injured nor nourished. It seems probable that this 



