The development of anti-horseradish peroxidase (HRP) producing cells was studied by electron microscopic immunocytochemistry in rat popliteal lymph node after a single injection of antigen with complete Freund‘s adjuvant. Specific antibody synthesis could be observed during the maturation stages of distinct cell types from the blast cell to the ergastoplasm-rich cell stage of either lymphocyte-like or reticulum-like cells. Cells with a lymphocytic aspect were tentatively called dense cell-type, and those of reticulum cell type were called clear cell-type.

Dense cell-types could be established without difficulty because of the high natural contrast of the cytoplasm and the nucleus. The cytoplasm always contained many ribisomes, typical of the lymphocytic series. From these cells, we distinguished reticulum cells and their pathways of differentiation due to their apparent reticulum-like characteristits. Reticulum cells showed poor fixation of cytoplasmic membranes, hence cellular filiations were always difficult to appreciate. The natural contrast of reticulum cell-types was typically weak. During their maturation stages, we observed grouped ribosomes in the cytoplasm as a typical feature becoming membrane bound, thus „clearing“ the cytoplasm.

Dense and clear cell-types may arise from different precursors. Several authors have stated that small circulating lymphocytes are the precursors of the plasmocytes, while others have claimed that plasmocytes may arise from reticulum cells.

Antibodies to HRP were first localized on Day 5 after HRP stimulation. They could be observed in some ergastoplasmic lamellae of plasma cells. The Golgi complex was often stained too. At the ultrastructural level, these cells were dense type mature plasma cells. From Day 6 typical blast cells became numerous. Some clear type plasmablasts and plasma cells were also found. On Day 10 and afterwards, increased numbers of both clear type and dense type anti-HRP producing cells occurred.