The Interleukin Network and Lymphoid Development

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Abstract
Lymphoid development differs sharply between the primary and secondary lymphoid organs. In the former, lymphocytes arise from precursors by antigen-independent processes under thymic or bone marrow microenvironmental influences and undergo extensive selective processes before being allowed to leave. In the latter, lymphocytes with receptors relevant to particular antigens undergo a second wave of proliferation and differentiation leading to the emergence of immunocytes with effector functions. Each of the two sets of events are profoundly dependent on cellular interactions. In the primary lymphoid organs, the "action" centres on stromal cell-lymphoid precursor interactions, and artificial systems permitting B cell formation are much more advanced than those for T cell development. For B cells, IL-7 and c-kit ligand (KL) are clearly important but so are as yet undefined stromal cell-derived activities. For thymic development, only fragments of the complex 3-week process of T cell formation can be mimicked in vitro and no IL has unequivocally been shown to be critical. Within the secondary lymphoid organs, where lymphocytes react to the antigenic universe, the key to regulation lies in interactions between accessory cells (dendritic cells, macrophages and their various relatives) T cells and B cells. Efforts to squeeze the relevant cytokines into sharp compartments such as activation factors, growth factors and differentiation factors have been largely unsuccessful.(ABSTRACT TRUNCATED AT 250 WORDS)