In Vivo Isotype Regulation of Humoral Responses to Dextran B1355 in BALB/c Mice Double‐Class IgG3/IgM Producers as a Function of Age

This report shows that, in 8‐ to 10‐month‐old BALB/c mice immunized intraperitoneally with dextran B1355, approximately 75% of IgG3 anti‐α(l→3) polyglucan (anti‐dex) plaque‐forming cells (PFC) detected in the spleen were identified as double‐Ig class producers secreting simultaneously IgG3 and IgM antibodies with the same specificity for the dex epitope. Under the same conditions of immunization, however, IgA anti‐dex PFC were mostly single‐class secretors. IgA PFC developed in the spleen in highest numbers (equal to IgM), but in Peyer's patches IgA FFC were sevenfold more numerous than IgM. Furthermore, spleen IgG3 anti‐dex PFC responses were low compared with spleen IgA and IgM anti‐dex PFC responses and appeared only late in ontogeny. The possibility is discussed whether a TH dependence of the IgA anti‐dex response and a TH‐independent generation of the IgG3 response are responsible for the different pattern of isotype expression.