The sense of place in the immune system.

This series of reviews examines the effect of differing tissue environments on the activity and functional capacity of cells in the immune system. From their origins as hematopoietic stem cells, throughout their development and as mature cells, cells of the immune system find themselves in distinct and highly specialized niches, and contact with antigen or inflammatory signals changes their phenotype, activity and trafficking. Two-photon microscopy has provided the first direct observations of living cells and their activation choreography in the tissue environment and will no doubt continue to provide greater understanding of cellular dynamics and immune function.