In many aspects, in vivo imaging is superior to other techniques, such as conventional histology, because it allows the stepwise process of leukocyte migration to be dissected. The potential uses of the in vivo imaging techniques have yet to be achieved. The transfer of lymphocytes with fluorescent dyes can be marked. APCs that ingest fluorescent antigen and use fluorescent antibodies specifically to label different types of APCs in the anterior segment can be identified. Combining these techniques could result in more exciting insights into uveitis and the mechanisms of the immune system. For example, one can confidently expect to be able to observe the extravasation of CFSE-marked DO11.10 T cells and their subsequent interaction with APCs containing red fluorescent OVA. The use of green fluorescent protein to measure the expression of various proteins has occurred in a variety of in vitro and in vivo models. To identify activated T cells specifically, heterozygous knock-in mice have been generated that have a cDNA for enhanced green fluorescent protein substituted for the IL-2 coding region on one allele [91]. Activated T cells from these IL-2-green fluorescent protein mice up-regulate the expression of the IL-2 gene and consequently synthesize the green fluorescent protein marker. The authors recently demonstrated the presence of these activated cells in vivo in the mouse iris [16]. With both the green fluorescent protein and the brighter yellow fluorescent protein the authors anticipate being able to detect in vivo the expression of various cytokines, such as IL-2, the Th1 cytokine IFN-gamma, or the Th2 cytokine IL-4, in T cells infiltrating the eye. First steps have been undertaken to establish in vivo imaging in human eyes. Toxicity of fluorescent dyes currently used for animal research and movement artifacts of the tissue under investigation amplified by the high magnification necessary for cell research, however, remain the major challenges to study leukocyte migration in human uveitis.