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Modification of immunologic response. IV. A factor in the serum of mice treated with streptomycin, its character and mode of action in vitro.

A factor contained in the serum of mice treated with streptomycin (SM) when added to spleen cell cultures stimulated the primary immunologic humoral response to sheep red blood cells (SRBC) in vitro. The magnitude of the immunologic response was measured as the number of specific antibody-producing cells. By the use of an immunoadsorbent, it was shown that the factor is not SM or a protein-SM complex remaining in trace amounts in the serum of mice treated with this antibiotic. Studies on the mechanism of stimulation by this factor showed that it reacts in a late stage of the immunologic response to already differentiated precursors of antibody-producing cells, and that higher concentrations of this substance added to cultures, instead of stimulating, inhibit producing of antibodies to SRBC. The mechanism of the stimulating action of this factor on antibody-producing cells is discussed.