Rabbits presumably sensitized to components of the streptococcal cell by natural contact with cross-reactive microbial antigens in their environment, display distinctive phases of evolving allergic response to an extract of streptococcal cells. Animals under 4 weeks of age show no gross allergic skin reaction; this is followed successively by age periods in which allergic reactions become most prominent at 4 hours, 23 hours, or 2 to 4 days after injection.

A relapsing nodular lesion, produced by cell wall components in the cell extract, occurs in neonatal rabbits which display no hypersensitivity, reaches a peak response in young animals during the period of most active growth, and decreases in severity in older rabbits while the degree of hypersensitivity is increasing. Relapses occur with greater frequency in neonatal than in older rabbits. Passive transfer of spleen cells or serum from sensitized adult rabbits has no influence on the nodular reaction in neonatal animals.

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