Intravenous inoculation of small doses of non-hemolytic streptococci into previously sensitized rabbits is usually followed by the appearance of perivascular cellular aggregates in lung and liver.

The characteristic cell in these aggregates is moderately large, with vesicular nucleus, prominent nucleoli, clumped chromatin, and basophilic cytoplasm. In addition, the lesions contain small lymphocytes and granulocytes.

This lesion is easily differentiated by architecture and cell content from normally occurring lymphoid aggregates, and from spontaneous rabbit hepatic cirrhosis.

This mononuclear response does not occur when the intravenous dose is large enough to cause death of the animal within 24 hours.

In spleen and lymph nodes the characteristic basophilic cells, which normally occur in these organs, are present in increased numbers.

Following intravenous treatment alone, or sensitization without intravenous treatment, the lesions occur much less frequently, and when present are smaller and more sparsely found.

Inasmuch as in the present series of experiments this lesion was not found in normal animals, and infrequently in those treated by the intravenous route alone, it is suggested that the preliminary sensitization serves to enhance the animal's reactivity to the antigen. In this way a small dose of bacteria is capable of eliciting the cellular phenomenon, which in unsensitized animals appears only when larger doses of antigen are administered over longer periods of time. Too large a dose of antigen, however, results in shock and cell death rather than proliferation.

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