It has been shown that the pneumococcus multiplying in the tissues of the immunized animal (horse) becomes attenuated: loses, in varying degrees, its virulence, capacity of capsule formation, susceptibility to phagocytosis, and type specificity. The antigenic activity as an immunizing agent and the production of "soluble specific substance" are also altered. In some instances, the typical pneumococcus characteristics may be quickly restored by one or two passages through a susceptible animal (mouse). In others, virulence is not recovered and the organism remains atypical.
Whether these changes are to be attributed to the specific action of immune bodies in the tissues, or are to be considered as the result of some biologic adaptive process to an adverse environment, has not been determined. Proof of the specific action of immune bodies is possibly open to question, whereas it is well known that virulence and with it some other characteristics are profoundly affected under unfavorable conditions in the absence of immune bodies, notably when the organism is grown at elevated temperatures or in certain unsuitable media.
These studies of pneumococci isolated from the infected immunized horse provide opportunities for further investigation of the significance of changes in virulence, type specificity, and formation of "soluble specific substances" in various forms of pneumococcus infection.