IMMUNIZATION OF RABBITS TO PRODUCE HIGH SERUM TITRES OF NEUTRALIZING ANTIBODIES AND IMMUNITY TO THE PARALYZING TOXIN OF IXODES HOLOCYCLUS

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Abstract
Rabbits have been immunized against the effects of the paralyzing toxin of the Australian paralysis tick Ixodes holocyclus by injecting them with preparations extracted from tick salivary glands. Immunized rabbits were able to withstand doses of toxin known to kill unimmunized rabbits. Neutralizing antibodies were detectable in serum after 2–4 doses of the crude extract or of the relatively pure antigenic fraction. When injections were continued at intervals of from 2–7 weeks, hyperimmunity was retained for at least 68 weeks. Hyperimmune serum, reaching a very high titre of neutralizing antibodies, was obtained after 3–6 injections. Titres tended to decline when boosting ceased, but after a 'rest period' high titres were restored by further boosting with normally lethal doses of toxin. No symptoms of tick paralysis developed despite low titres prior to boosting. Thus, once hyperimmunity had been established, high titres of circulating antibodies were not immediately essential for immunity to tick paralysis. An IgG fraction was obtained from rabbit serum using a Protein A-Sepharose method; 33.4 g of IgG protein fully neutralized, and 19.5 g IgG half neutralized, 1 g of crude toxin protein. This procedure with rabbits may permit the production of a purified tick-paralysis antitoxin more suitable for human use than the existing antitoxin based on canine hyperimmune serum.

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