The number of mature cytoplasmic immunoglobulin containing cells (C-Ig cells) in various lymphoid organs of the mouse was studied as a function of antigenic load and the presence of T cells. Both antigen and T-cell deprivation decreased the number of C-Ig cells (plasma blasts and plasma cells). The absolute C-Ig cell numbers were minimal in germfree nude mice, intermediate in specific pathogen-free (SPF) nude mice and in germ-free thymus-bearing mice, and the largest in SPF thymus-bearing mice. The bone marrow was the organ where this picture was most strikingly examplified. In germfree C3H mice raised on a synthetic diet the bone marrow of 3 out of 4 mice tested did not show any C-Ig cell. None of them had C-Ig cells in the mesenteric lymph nodes. The Ig class distribution of the C-Ig cells and the incidence of C-Ig cells positive for more than one Ig heavy-chain isotype (‘double producers’) were also highly dependent upon the antigenic load and the presence of T cells. C-IgM cells were well represented in all groups of mice. Appreciable numbers of C-IgA cells were only found in SPF thymus-bearing mice. The absolute number, but not the relative number, of C-IgG cells followed the same pattern. The percentages of double producers in the various lymphoid organs were largest in germfree nude mice, intermediate in SPF nude mice and in germfree thymus-bearing mice, and minimal in SPF thymus-bearing mice. From these studies we conclude that the background synthesis of IgG and IgA in not intentionally immunized mice is virtually completely dependent on stimulation of the immune system by external antigens, in contrast to the production of
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