Spatiotemporal Basis of Innate and Adaptive Immunity in Secondary Lymphoid Tissue*

Secondary lymphoid tissues are the sites of both innate and adaptive host defense. Aside from the relatively static nonhematopoietic stromal elements and some macrophages and dendritic cells, most of the cells in these tissues are in constant movement, but the organs maintain a defined microanatomy with preferred locations for the bulk of T cells, B cells, and other lymphocytes and subsets of myeloid cells. Here we describe both the cell dynamics and spatial organization of lymph nodes and review how both physical features and molecular cues guide cell movement to optimize host defense. We emphasize the role of locality in improving the efficiency of a system requiring rare cells to find each other and interact productively through membrane-bound or short-range secreted mediators and highlight how changes in steady-state cell positioning during an infectious challenge contribute to rapid generation of productive responses.