Isotypes and specificities of immunoglobulins produced by germ‐free mice fed chemically defined ultrafiltered “antigen‐free” diet

The regulation of the “spontaneously” occurring (“background”) Ig synthesis of mice has been studied by determining the numbers of IgM‐, IgG‐ and IgA‐secreting cells and a part of the IgM antibody‐specificity repertoire in spleen, bone marrow (BM) and mesenteric lymph nodes (MLN) of conventional and “antigen‐free” mice. These antigen‐free mice were germ‐free raised and fed an ultrafiltered solution of chemically defined low molecular weight nutrients, and thus devoid of exogenous antigenic stimulation. The secretion of IgM, IgG and IgA by spleen, BM and MLN cells was assessed in the protein A plaque assay, while specific IgM antibody‐secreting cells were detected by plaque assays specific for differently haptenated sheep red blood cells.