While preliminary treatment with culture filtrates or dead cells of the extremely virulent strain A gave rise to varying degrees of immunity, the exudative lung lesions developing after tracheal injection with live organisms of the same virulent strain were not strikingly increased in any group or series of these rabbits. Despite carefully graduated dosage in the preliminary treatment, none of the animals developed symptoms of a definitely anaphylactic nature.

In similar experiments following sensitization with the attenuated avirulent culture AA, tracheal injection of virulent or avirulent organisms of the same strain failed to incite any definite increase in the exudative lung reaction. In none of the rabbits were symptoms resembling anaphylaxis noted. The immunity which was induced by the larger sensitizing doses of culture filtrates of the strain in the virulent state was lacking when similar doses of the culture filtrates of organisms in the non-virulent state were used.

Extensive lesions developed in both sensitized and unsensitized rabbits when the strain used in the tracheal injection was one apparently combining moderate virulence with exceptional toxicity, indicating that the exudative lung reaction was one of adjustment rather than of acquired hypersusceptibility.

When, in the experiments, small amounts of sera from normal rabbits, or from animals immunized to culture filtrates, were added to the culture before tracheal injection, an increased fibrinous lung reaction was frequently found.

The present study would seem to give some ground for the view that while in pneumonia a hypersensitive condition probably takes some part in the inception of the infection, the subsequent development of the diffuse exudative reaction in the lung is not directly due to an acquired hypersusceptibility, but to intrinsic qualities possessed by the pneumococcus itself.

This study was carried on in the Department of Bacteriology of the College of Physicians and Surgeons of Columbia University, New York, and later, through the courtesy of Dr. Charles Norris, in the Bellevue Hospital Laboratories. It was completed in the Laboratories of the New York State Department of Health at Albany. I am greatly indebted to Dr. Augustus Wadsworth for his advice and criticism.

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