Increase in intracellular immunoglobulins in the majority of splenic lymphoid cells after primary immunization

The amount of immunoglobulin (Ig) in individual mouse spleen lymphocytes was determined by quantitative microspectrofluorimetry after “staining” with fluorescein-labeled anti-Ig antibodies. During a primary immune response against sheep erythrocytes, maximally 0.015 % of the cells produced direct hemolytic plaques, but close to 100 % of lymphoid cells displayed an increase in their intracellular Ig on day 3 to 4 after immunization. This increase consisted of IgM, later to be followed in a large fraction (up to 80 %) of the cells by IgGl. A positive correlation was found between rate of protein synthesis and fluorescence intensity in the single cell. Preliminary data suggested that activated thymus-derived cells displayed an increase in an intracellular material, cross-reactive with serum IgM. Attempts to demonstrate that the intracellular increase was due to endocytosis of Ig in various forms failed in several model systems. The reported phenomenon might demonstrate the production of a substance, capable of alerting the majority of all lymphoid cells during a primary immune response, into increased Ig synthesis.