A prevailing explanation for this phenomenon concerns the aberrant inflammatory cytokine profile of individuals with schizophrenia, which features increased levels of markers of immune cell activation, including soluble IL-2 receptor and increased circulating levels of pro-inflammatory cytokines, such as IL-6. Notably, high blood concentrations of IL-1 receptor antagonist, an anti-inflammatory cytokine that could have protective effects against RA, are also found in individuals with schizophrenia.

Publication reference : Sturgeon JA, Finan PH, Zautra AJ. Affective disturbance in rheumatoid arthritis: psychological and disease-related pathways. Nat Rev Rheumatol. 2016;12(9):532–542
Reference URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5449457/