In vitro antigen stimulation of DNA synthesis in lymph node cultures from immunized guinea pigs can be obtained with very low (10–4 µg/ml) antigen concentrations in the culture fluid. Immunization with low doses of DNP-GPA leads to a cell population capable of being stimulated, on the average, by low concentration of antigen whereas immunization with large antigen doses results in a sensitive cell population requiring, on the average, high antigen concentrations for stimulation. These findings correlate well with the affinity for hapten of the serum antibodies produced by these guinea pigs.

Both delayed reactions in vivo and DNA synthesis in vitro can be stimulated by hapten conjugated to proteins different from that used in primary immunization. However the immunizing conjugate is much more effective in terms of antigen concentration required for a given response.

These results can be understood in terms of a thermodynamically driven interaction of antigen (or "processed" antigen) with cell-associated antibody.

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