NATURAL IMMUNITY 267 



of the blood or the toxic elements of the blood are the source of venom, Flex- 

 ner and Noguchi distinguished the differences in these two sets of active 

 principles by their capability to unite with or to be activated by their homolo- 

 gous and heterogeneous complements. According to these authors venom 

 lysins are capable of being activated by isocomplements as well as hetero- 

 complements, while the amboceptors of the snake serums are active only in 

 the presence of their own complements. This explains why the snake serum 

 loses its toxicity when heated to 56 C. or above; here the inactivation is due 

 to the disappearance of suitable activators — isocomplements in this case. 

 At present our knowledge concerning the venom activators is so enlarged 

 that the so-called heterocomplements, in Flexner and Noguchi's sense, com- 

 prise lecithin, certain fatty substances, and also serum complements. 



It becomes probable that Calmette's view on the relation of blood toxicity 

 and venom toxicity was a proper one. 



Stephens * and Noc found that Calmette's antivenin neutralizes the hsemo- 

 lytic principle of snake serums, especially that of Cobra. 



Theoretically considered, natural immunity must be regarded as the ex- 

 pression of combination of many factors. It is seldom that the blood of an 

 animal refractory to the effect of a toxin contains a definite anti-substance 

 comparable to the product of artificial immunization known as immune body 

 or antitoxin. In the cases of poisons, such as saponin and other glucosids, 

 the blood may contain a definite substance (like cholesterin) capable of direct 

 neutralization, but in other cases — such as certain alkaloids — toleration 

 through the repeated introduction of these bodies into an animal body may 

 be attained without inducing the formation of any definite anti-body. In 

 still other cases, normal serums often contain substances similar to real 

 antitoxins, as in horse serum for tetanus toxin. 



Again to-day we find very instructive instances of another set of phenomena 

 pointing to the cellular and vital processes of immunity, either natural or 

 acquired — namely, the r61es of phagocytosis advocated so long by Metchni- 

 koff and his collaborators. In this instance the substances called opsonins 

 by Wright and cytotropic substances by Neufeld must be regarded as the cause 

 of natural as well as acquired immunity. 



Still other examples of natural immunity are the cases of chicken against 

 tetanus, and hen and tortoise against abrin. There are no antitoxic prop- 

 erties in these bloods. This may be due to the absence of suitable receptors 

 in the sense of Ehrlich's side-chain theory. 



In considering the nature of natural immunity in snakes against their own 

 and alien venoms, we must take the above factors into account before we can 

 reach a conclusion. I am inclined to think that the phenomenon is the 

 expression of not one single circumstance, but of several circumstances here 

 enumerated. The lack of suitable receptors especially seems to be playing 

 a dominant part. 



1 Stephens. On the hemolytic action of snake toxins and snake sera. Jour, of Path, and Bacterid., 

 1900, VI, 373. 



