Photos: Ian Parker & Mark Miller
 SHALL WE DANCE? Key immune players cut a rug in a lymph node. Shown are T cells (green), B cells (red), and dendritic cells (blue). Inset: T cells and reticular fibers (red). Both pictures were acquired using TPLSM.

Using a technique called two-photon laser- scanning microscopy (TPLSM) researchers can visualize, in three dimensions, the cellular waltzes by which the mammalian immune system develops and reacts to infection. The technique enables two low-energy photons to converge additively, exciting fluorophores only at the focal plane, which can be more than 300 µm beneath the surface of the tissue being examined.1 TPLSM represents a vast improvement over confocal laser techniques, which use single high-energy photons and cause photobleaching and phototoxicity, and cannot penetrate tissue. First pioneered in 1990, TPLSM began creeping into bio- logical research by mid-to-late decade; improvements to the reliability and usability of femtosecond...

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