Among the experimental findings reported in this paper to which we wish to give particular emphasis are the following:
1. The results which follow the intravenous injection into rabbits of two strains of Pneumococcus Type III of different degrees of virulence vary with the state of the capsule. Thus when this structure is completely developed both remain in the blood. A culture of either strain begins to become susceptible to the blood-clearing mechanism contemporaneously with the onset of capsular degeneration and the initiation of other concomitant changes at the surface of the organism (cf Paper II), which occur much earlier with the less virulent strain.
2. When, in either case, removal from the blood stream occurs, this is effected by the phagocytic cells of the body. There is no suggestion that a new or unknown mechanism is involved. The greatest share of the burden is borne by the fixed phagocytic cells of the liver and spleen, and to a less extent by those of the lung and bone marrow. Nevertheless, it has been demonstrated that the polymorphonuclear leucocytes may also participate.
3. Phagocytosis by the leucocytes of the normal animal either in intro or in vivo has been observed only at such a time as the capsule has become impaired. Ingestion of the organisms by the fixed tissue cells appears also to be effective only under the same condition and is accordingly observed with much younger cultures of the less virulent strain.
4. Following their removal from the blood and their accumulation within the fixed phagocytes of the organs, destruction of most of the cocci proceeds within 2 to 4 hours. Both strains are destroyed provided they are in the state favorable to phagocytic attack.
5. Evidence has been presented which indicates that just as in vitro, so in a local area of inflammation within the body, aging with attendant capsular loss and increasing susceptibility to phagocytosis may take place.
6. With organisms from either strain a variable period of lag follows their injection into the blood stream, even when they are introduced in a state of active multiplication and complete encapsulation.
7. Differences in virulence for rabbits of two strains of Pneumococcus Type III do not imply that this animal possesses a defensive mechanism which is absent in other species, since it has been possible to demonstrate similar differences when the organisms are injected intravenously into mice. This fact indicates that the factors determining the degree of virulence of these strains are to be sought in the organisms themselves, rather than in the kind of host.