NATURAL RESISTANCE TO DISEASE—-FLEXNER. TS 
contained within it along with parasites. In fact, this method of 
conjoint vaccination with the parasite of the disease and the blood 
containing immunity principles is one that offers a considerable field 
of practical application. On the one hand, there is accomplished a 
passive immunization of the body that becomes operative immedi- 
ately, and, on the other hand, a vaccination that after the usual inter- 
val leads to the production of a state of active immunity that rises 
to a higher level and is far more enduring than the passive state. 
Incidentally we have discovered from this process of mixed or con- 
joint vaccination that immune sera prepared for bacteria or other 
parasites which are not toxin producers in the manner of the diph- 
theria bacillus, but which contain endotoxin, act not especially by 
neutralizing toxins, or by destroying outright the bacteria, but by 
exercising an efficient protective control over the injury which these 
parasites or their poisons tend to inflict on certain sensitive body cells. 
For example, if cattle are inoculated on one side of the body with 
virulent blood from animals dying of rinderpest, and on the other 
side with blood serum taken from animals that have recoverd from 
the disease and subsequently have had their immunity intensified by 
injections of highly virulent blood, the cattle so vaccinated will de- 
velop rinderpest in a mild form and will subsequently on recovery 
be also immune; and yet during the process of immunization their 
blood contains highly virulent parasites, so that if a little of it be 
introduced into nonprotected and healthy cattle, they will be given 
rinderpest and will die of it. 
The reaction of the body to the bacterial vaccines injected is out 
of proportion to the quantity of culture introduced. Thus two milli- 
grams of dead cholera bacilli injected under the skin of human beings 
will yield enough of the specific immunity substance for these bacilli 
to bring about the destruction of 60,000 or more milligrams of the 
culture. There can be, therefore, no direct tranformation of the 
cholera bacilli into immunity bodies, but they must exert a stimulus 
on certain cell-functions through which the immunity principles are 
produced; and the quantity of their formation depends not on the 
weight of crude bacilli introduced, but on the strength of the stimu- 
lus impressed upon the sensitive cells to which they react in a specific 
and remarkable manner. 
Is it possible in the course of an established infection to reinforce 
the resistance of the body? I have already stated that it is not prac- 
ticable to bring out at the height of an infection an efficient height- 
ened reaction of physiological resistance; but from this it does not 
follow that under these conditions a special form of immunity reac- 
tion may not be elicited. The tuberculin reaction, or that part of 
it which is specific, may be cited as an example of this kind of rein- 
forcement; and whatever there is of value in the treatment of infec- 
