There is growing evidence that dendritic cells, the major antigen-presenting cells and T-cell activators, have a broad effect on peripheral T-cell tolerance and regulation of immunity. Very recently, a new feature of regulatory antigen-presenting cells was observed. Certain dendritic cells, monocytes, and macrophages express the enzyme indoleamine 2,3-dioxygenase, and thus because of enhanced degradation of the essential amino acid tryptophan, they modulate T-cell activity in specific local tissue environments. In this review we discuss the various and apparently disparate effects of indoleamine 2,3-dioxygenase induction in cells of the immune system. We place current knowledge about this mechanism in the context of atopy. We introduce the hypothesis that tryptophan degradation might add to the ability to control and downregulate allergen-specific T-cell responses in atopic individuals.