Based on the results of clinical and preclinical studies, it has been proposed that the switch from sickness to the cognitive/affective symptoms of depression is mediated by immune-dependent activation of the tryptophan metabolizing enzyme, indoleamine 2,3 dioxygenase (IDO1) (Dantzer et al., 2008) (Fig. 2). Here, IDO1 is linked to depressive symptom measurement.