The mechanism of action of tacrolimus in atopic dermatitis is not fully understood. Whilethe following have been observed, the clinical significance of these observations in atopicdermatitis is not known. Via its binding to a specific cytoplasmic immunophilin (FKBP12),tacrolimus inhibits calcium-dependent signal transduction pathways in T cells, therebypreventing the transcription and synthesis of IL-2, IL-3, IL-4, IL-5 and other cytokines such asGM-CSF, TNF-α and IFN-γ. In patients with atopic dermatitis, improvement of skin lesionsduring treatment with tacrolimus ointment was associated with reduced Fc receptorexpression on Langerhans cells and a reduction of their hyperstimulatory activity towards Tcells. Tacrolimus ointment does not affect collagen synthesis in humans.