[Non lymphoid cells of the peripheral lymphoid organs. Recent morphological progress and current understanding...

Peripheral lymphoid organs have two groups of non lymphoid cells: that of macrophages which belongs to R. van Furth's system of mononuclear phagocytes and the one made of elements for long labelled "reticular cells". The use of electron microscopy has permitted a good understanding of these elements and a recognition of ubiquitous cells (fibroblasts, endothelial cells) and specific cells to the lymph node which seem to play a very important role in the immune reactions (interdigitated cells, reticulum cells, dendritic cells). We describe their light and electron microscopical features and histoenzymatic characters, and present the actual concept of their origin and function. The latest discoveries on their classification are reviewed and have permitted to distinguish three categories of non lymphoid cells: first the group of reticulum cells (reticulum cells and dendritic cells), second that of mononuclear phagocytes (macrophages and interdigitated cells) and third that of endothelial cells. Other elements of questionable existence or insufficiently known do not appear in this classification: dark cells, Steinman and Cohn's dendritic cells and cells containing Birbeck's granules.