Given that the lung has at least two resident cell types that constitutively express IDO protein (endothelial cells and lung myeloid dendritic cells [DCs]), and two other types that can be recruited in larger numbers during infections or allergic inflammation (plasmacytoid DCs and eosinophils), it appears that the function of IDO is stimulatory or inhibitory depending on the target cell, the stimulus, and specific model [38]. This evidence concerns the gene IDO1 and infection.