In the pathogenesis of depressive symptoms the neurotransmitter serotonin plays an important role--although the underlining mechanisms are still not clear. The synthesis of serotonin is dependent on the availability of tryptophan--an amino acid that is linked to the immune system by its catabolism via the enzyme indoleamine-2,3-dioxygenase (IDO). Based on this connection research approaches addressing different clinical conditions with depressive symptoms and immunological involvement have been considered. This review provides an overview on the latest research in the field.