Spontaneous active immunity to the phenomenon of local skin reactivity to bacterial filtrates has been demonstrated. In one experiment, the immunity was non-specific, while in others it appeared limited to one or two bacterial species.

Intradermal vaccination of rabbits with bacterial filtrates induced active immunity to the phenomenon. The specificity of this immunity was tested with graded amounts of toxic factors. The combined intradermal and intravenous immunization in these experiments elicited two types of response:—

1. Specific immunity, which was obtained in rabbits highly susceptible to the phenomenon (i.e., showing reactions with 1 reacting unit).

2. Non-specific immunity which was elicited in rabbits with partial spontaneous immunity (i.e. showing no reactions with 1 reacting unit, but susceptible to 15 units 1 week later). Comparative studies on the reacting potency of "agar washings" and disintegrates (Besredka's method, freezing and thawing method) showed that the first washings of bacterial cells are considerably stronger than subsequent washings and disintegrates. These facts demonstrate once again the extracellular nature of the material necessary for the phenomenon.

Tentative explanations of the mechanisms involved in the specific and non-specific immunity to the phenomenon are discussed.

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