Cellular Differentiation of Lymphoid Subpopulations and Their Microenvironments in the Human Thymus

The technological development of monoclonal antibody production has opened up a somewhat unexpected door to the stage of molecular histo(patho)logy. These pure reagents recognize, through specific antibody-antigen reactions, epitopes of immunogenic macromolecules which in certain cases, such as OKT3 (KUNG et al. 1979), appear to be fairly lineage specific. In other cases, the antibodies may react in certain characteristic combinations with a number of cell types that belong to different lineages. In these cases, the. antibodies frequently label identical macromolecules (or at least major fragments of macro- molecules), so that these findings are not merely due to shared epitopes on otherwise dissimilar molecules (cf. PLATT et al. 1983; HAYNES et al. 1983). The important point about these observations is that the tissue distribution of these conveniently identified antigens may not always be purely accidental: the recognized antigens could represent functionally important macromolecules or reflect the common origin of sometimes widely distributed cells. It is intriguing that, for example, precursor cells of the B-lymphoid lineage and glomerular epithelium share at least five differentiation antigens (perhaps of common mesodermal origin; PLATT et al. 1983) and the glycolipid molecules recognized by A2B5 antibody are found on cells of secretory function, apparently including thymic medullary epithelium, as well as on cells of neural crest origin (HAYNES et al. 1983; see also below).