The expanding fields of psychoneuroimmunology, and lately also immunopsychiatry, has increased our knowledge about the complex and reciprocal interactions between the immune system and the brain. The detected immunological abnormalities in subgroups of psychiatric patients have increased our understanding of precise immunological mechanisms underlying and contributing to the etiopathology of psychiatric disorders. The research developments towards identifying immune-related subgroups of mental disorders with distinct features and symptom profiles would disentangle some of the heterogeneity inherent in mental disorders. Identification and validation of specific immune-related biomarkers in cerebrospinal fluid and blood aid in the differential diagnosis, while ongoing microbiome research and brain imaging of neuroimmunological processes could further contribute to the etiopathology of mental disorders. Immune markers can assist in the treatment decisions, where immune modulating treatments for subgroups of mental disorders with underlying immunological disease activity could improve the current treatment of mental disorders at least for subgroups of patients.