Rabbits were immunized by intraperitoneal injections of DNP-Ascaris antigen precipitated with aluminum hydroxide gel to form reaginic antibodies. Mesenteric lymph node cells from the primed rabbits were stimulated in vitro by incubating with homologous antigen for 24 hr and then cultured for 6 days in the absence of antigen. Measurements of IgG and reaginic antibodies in culture fluids showed that the lymph node cells from the rabbits which formed reaginic antibodies after primary immunization produced IgG antibodies. In vitro reaginic antibody formation was generally observed when donors of lymph node cells showed both primary and secondary reaginic antibody responses after the primary and booster injections. The titer of reaginic antibody in the culture fluid, as determined by the PCA reaction, did not correlate with the concentration of IgG antibodies in the fluid, or with the reaginic antibody titer of the serum of donors at the time of sacrifice. The molecular size of reaginic antibody in the culture fluids was intermediate between those of IgM and IgG antibodies. The skin-sensitizing activity of the antibody was completely lost by treatment with 0.01 M dithiothreitol followed by alkylation and slightly diminished after heating at 56°C for 2 hr. Such properties of the reaginic antibodies in the culture fluid were comparable to those of IgE antibodies in the serum.

It was found that a removal of most of the adherent cells in lymph node cell suspension before antigen stimulation in vitro did not have a significant effect on the formation of IgG, IgM and IgE antibodies in the culture. An addition of monospecific anti-IgG with antigen for in vitro stimulation suppressed in vitro IgG and IgM antibody formation but not the IgE antibody formation.

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