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The relative amounts of Ig kappa and Ig lambda1 anti-2,4-dinitrophenyl antibodies were measured at various times after immunizing mice with prototype thymus-dependent (TD), thymus-independent type 1 (TI-1) and thymus-independent type 2 (TI-2) antigens. Similar amounts of Ig lambda1 were produced after TD and TI-2 immunization and somewhat less was produced after a TI-1 stimulus. In contrast, Ig kappa levels were much greater after TD than after TI-1 or TI-2 antigen. The amount of light chain isotype produced appeared to depend on the molecular form in which the hapten was presented, although possible adjuvant effects were not ruled out. Levels of Ig kappa and Ig lambda present in nonimmune sera were measured in normal, xid and nude mice. The kappa/lambda ratio was higher in xid than in normal mice and the difference was demonstrated by F1 analysis to be due to an X-linked gene. Conversely, the kappa/lambda ratio was lower in nude than in normal mice. This was true for the CBA/Tufts (Ighj), CBA.Ighb and C57BL/10 strains. However, there were no detectable differences in the relative frequencies of surface Ig kappa- and Ig lambda-bearing B cells in adult CBA/Tufts, CBA/N and nude mice. Hence, serum ratios may reflect differences at the level of B cell triggering. Two possible explanations for these differences are discussed. Ig kappa and Ig lambda may be expressed on functionally distinct B cell subsets. (For instance Ig lambda-producing cells might be readily triggered by T1 antigens whereas Ig kappa-producing cells are more dependent on T cell signals. Such functional subsets could be determined by light chain expression). Alternatively, cells producing Ig kappa antibody are selected for because they have a higher affinity for antigen. If so, triggering of cells producing high affinity Ig kappa or their subsequent selection is T cell-dependent.

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