Melbourne JK, et al. - In psychotic disorders such as schizophrenia and bipolar disorder with psychosis, evidence suggests altered JAK-STAT1 pathway activity in peripheral blood cells. In view of the observation that activation of this pathway results in increased expression of complement component 4A (C4A), researchers here determined mRNA expression of C4A in peripheral blood cells from participants with schizophrenia, bipolar disorder and controls. as per finding C4A mRNA expression alone is positively associated with psychotic symptomatology, specifically the presence and severity of delusions. These findings seem noteworthy given recent findings that show a critical role for complement proteins in synaptic pruning, alterations of which are proposed to contribute to psychopathology in psychosis.