Associations between depression severity and immune activity in children and adolescents.

Numerous clinical and epidemiological studies have demonstrated altered immune activity in adult depression patients, yet comparable data in youth are scarce. This study investigated the relationship between depression severity and peripheral immune measures in a clinical sample of children and adolescents. We analyzed 1198 blood samples from 819 patients (age range: 8-18 years) diagnosed with mild, moderate, or severe depressive disorder according to ICD-10 criteria. Immune cell counts, immune cell ratios, and C-reactive protein were quantified from the samples. Associations between immune measures and depression severity were examined using multilevel regression models. Patients' age moderated the associations between depression severity and immune measures, with higher severity being associated with increased cell counts of monocytes, eosinophils, and basophils in older patients. In longitudinal analyses of patients with data from multiple timepoints, we found higher CRP levels to predict more severe depression over time. Our findings suggest that immune dysregulation in youth depression may not fully follow patterns established in adult populations, but they progressively approximates them as children grow older. This underscores the importance of conducting research on immunological changes in depression in youth and over the lifespan.