Two-photon tissue imaging: seeing the immune system in a fresh light

Abstract
Many lymphocyte functions, such as antigen recognition, take place deep in densely populated lymphoid organs. Because direct in vivo observation was not possible, the dynamics of immune-cell interactions have been inferred or extrapolated from in vitro studies. Two-photon fluorescence excitation uses extremely brief (in vivo setting and to reveal in detail the cellular collaborations that underlie the immune response.