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 heemo- 
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 rdles 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. 

1Stephens. On the hemolytic action of snake toxins and snake sera. Jour. of Path. and Bacteriol., 
1900, VI, 273. 
