Temporal relationship between primary injection and cyclophosphamide administration necessary for the inhibition of the priming for a secondary response.

Using the mouse PFC anti-SRBC response, the temporal relationship between primary antigen dose and cyclophosphamide (CP) administration necessary for the inhibition of priming for the secondary IgG response was studied. The administration of CP 1 h after the primary or secondary antigen injection inhibits the responses in both cases. The administration of CP 1 h after the primary antigen injection does not inhibit priming for the secondary response; this priming is inhibited if CP is administered at 12 h or more, and the maximal degree of inhibition is induced when CP is administered 7 or 9 days after the primary antigen injection. Therefore, the proliferation stage of mature B cells to IgM secretion is not necessary for this priming. It is suggested that the switch from IgM to IgG takes place in a proliferative stage of B-cell precursors in presence of the antigen.