From the results of this study of the action of immune sera on pneumococcus infection it is evident that immune sera vary greatly in their curative value. Immune sera possess protective action, but protective action is not necessarily indicative of curative action.
Treatment with the serum of normal rabbits may prolong the course of pneumococcus infection in the rabbit. This action, however, is slight and not always manifest. Sera from animals immunized with dead pneumococcus cells which had been washed free from their products, failed to exert materially greater curative action than normal sera. Sera from animals immunized with culture filtrates free from pneumococcus cells possessed, in some instances, a slight curative value, but often this curative action was not apparent.
In animals actively immunized, however, the presence of an immunity to culture filtrates was readily demonstrated. In the immunity produced by injections of dead culture material the strength was not sufficiently exalted for the sera to possess a practical curative value.
It was only after immunization with virulent living cultures that the blood serum acquired marked curative action. After pneumococcus infection in the rabbit had become established, treatment with this serum induced crisis and cured the animals.
From the results of the study of the mechanism of recovery it is evident that, despite the fact that virulent pneumococci are singularly insusceptible to the action of immune sera in the test-tube, pneumococcus infection nevertheless conforms to the general law of infection.
Diphtheria and tetanus organisms give rise to powerful toxins, but the parasitism of these organisms is slight and their development is localized. Diseases produced by these organisms are toxemias and neutralization of their toxins by antitoxin puts an end to the disease.
The pneumococcus gives rise to toxic substances which are less active or are active only in the body tissues, but the parasitism of this organism is marked and its development is rarely localized. Nevertheless, the manifestations of the disease arise from the action of the bacterial poisons on the tissues. The neutralization of the pneumococcus poisons by immune serum puts an end to the symptoms of the disease, but the pneumococci survive as harmless parasites until destroyed by lysis or phagocytosis.
The neutralization of the pneumococcus poison may take place suddenly and completely as in crises; or, it may be incomplete with exacerbations of infection, as in lysis. Crisis, as it occurs in the lobar pneumonia of man and in the bacteriemia of the rabbit, is simply one phase of recovery, and recovery does not differ fundamentally, whether it is sudden and complete as in crisis, or incomplete and prolonged as in lysis, or whether the pneumococci are destroyed by lysis extracellularly as in the rabbit, or intracellularly as in the phagocytosis of the dog and man.
Since the recovery of animals from pneumococcus infection differs in no essential from that of man, since the unaided protective mechanism of man as compared with that of susceptible animals is exceptionally efficient, and since it is possible by treatment with sera from animals highly immunized with living cultures of virulent pneumococci to cure pneumococcus infection in the most susceptible animals, it is difficult to conceive of the infection in man failing to yield similarly to the administration of such sera.