142 THE DIFFERENT TYPES OF IMMUNITY 



Different possibilities, of course, suggest themselves. Since a toxic 

 effect presupposes the existence on the part of some of the body 

 cells of special molecular groups with which the toxins can combine, 

 it stands to reason that a natural absence of such groups must lead 

 to natural immunity so far as that special toxin is concerned. But 

 if this be the case then the formation of a corresponding antitoxin 

 should not be possible. By this criterion, then, we can test any case 

 that might suggest itself as belonging to this order. Metschnikoff 

 has pointed out that certain reptiles, and notably the turtle, are 

 naturally absolutely immune to tetanus toxin; no matter whether 

 the animals be kept at the ordinary temperature of the aquarium, 

 or at 37 C., following an injection of the toxin, their blood remains 

 highly toxic for mice, even for several months. Coincidently he 

 found that there was not the slightest formation of antitoxin. This 

 example then illustrates especially well the actual existence of a 

 theoretically possible form of immunity due to absence of suitable 

 receptors. 



A second possibility suggests itself if we bear in mind that not 

 all cells which may possess suitable combining groups for a toxin 

 molecule are necessarily deleteriously affected by such a union. 

 In such an event we should expect absence of toxic effect associated 

 with the production of antitoxin, for we have seen that the latter 

 can take place perfectly well even though the specific action of the 

 toxophoric group is eliminated. That this may actually occur in 

 nature is well shown in the case of the American alligator, which 

 is as resistant to the action of the tetanus toxin as is the turtle, 

 but which, unlike the latter, furnishes an abundant amount of 

 corresponding antitoxin. That the toxin in this case is actually 

 bound by the cells is also shown by the fact that, contrary to what 

 we have noted in the turtle, it rapidly disappears from the circu- 

 lation. In such a case the immunity is evidently not due to absence 

 of suitable receptors, but to an insusceptibility on the part of the 

 binding cells. 



Still another possibility would exist, if both susceptible and in- 

 susceptible cells were present in the body, and if the latter possessed 

 a greater affinity for the toxin than the former. In such a case we 

 should theoretically expect active antitoxin formation, immunity 

 to small doses of the toxin, but absence of immunity to a larger 

 dose, the result, moreover, varying with the point at which the 



