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Lymph node macrophage heterogeneity: the phenotypic and functional characterization of two distinct populations of macrophages from rat lymph node

The isolation and characterization of lymph node macrophages (MΦ) has shown a hitherto unknown heterogeneity. Two types of MΦ were distinguished by morphology, monoclonal antibody staining and functional assays. The type I MΦ failed to express surface Ia even when activated, a characteristic which has only previously been reported for splenic marginal zone MΦ despite studies suggesting an antigen presentation role for the MΦ, the failure to express surface Ia would seem to eliminate an interaction with T helper cells for the type I MΦ in the lymph node. In contrast, the type I MΦ, other characteristics of clustering with activated B cells in vitro, the colocalization of the type I MΦ and activated B cells in situ, the specific uptake of thymus‐independent type 2 antigens and the failure to undergo respiratory burst activity all suggest a MΦ‐B cell interaction, possibly of a trophic nature.