FACTS AND PROBLEMS OF IMMUNITY 319 



may thus become a menace to the system at large by liberating ^ison- 

 ous bodies from comparatively harmless compounds. 



Thus, in many instances, it seems we are probably dealing with an 

 immunity, a large part of the mechanism of which is intracellular, not 

 only in the sense of phagocytosis and digestion, but in the neutralization 

 or destruction of poisons which arise from the disintegration of the bac- 

 teria and their products — a mechanism in which the protecting cells 

 must intervene and, largely unaided by antitoxic bodies in the plasma, 

 neutralize or destroy within themselves the poisonous products of the in- 

 vading microorganisms. 



It was this thought which suggested the idea of treating infections 

 with the extract of leucocytes, and thus aiding the phagocytes by 

 furnishing them as directly as possible M'ith the weapons which they 

 use in their fight with invading microorganisms, and also to protect 

 them and the cells of specialized function from destruction and give 

 them an opportunity to recuperate and carry on successfully their 

 struggle against the invading germs. 



The treatment of infections with vaccines also is based upon the 

 recognition of the necessity of the direct intervention of phagocytes 

 in the cure of certain bacterial diseases and is, as we have seen, an 

 endeavor to stimulate the production of substances facilitating the 

 ingestion of the organisms b}' the phagocytes. 



Finally, it must be remembered that while animal experiments are 

 necessary, and often extremely instructive, one can, nevertheless, not 

 always argue directly from these to occurrences in man. An injection 

 disease is not an infectious disease, and we are dealing usually with con- 

 ditions in man which are at least not entirely analogous to artificial in- 

 fections in animals. Artificial infections are usually accomplished by an 

 abrupt introduction of a large quantity of infecting germs and their 

 products ; the animal powers of resistance are often immediately and se- 

 verely taxed; the incubation period thus artificially shortened; and the 

 germs themselves, being present in large numbers, are not subjected to 

 such a searching elimination as is usually the case with the few organisms 

 gaining a foothold by the natural channels of infection. This difference 

 is most marked in septicemias, in which, in animal experiments, the or- 

 ganisms have been introduced directly into the circulation in quantities 

 sufficient to bring about a very rapid poisoning and overwhelming of the 

 animal, with probably only a very partial adaptation of the bacteria 

 to the animal agents of resistance. On the other hand, the septicemic 

 invasion in man most often follows the adaptation of the germs in some 



