C4A and schizophrenia: Several studies in the psychosis and schizophrenia field have now measured complement C4 or C4 fragments in biological samples, such as blood plasma or cerebrospinal fluid (CSF) (Gangadin et al., 2023; Gracias et al., 2022; Hatzimanolis et al., 2022; Kalinowski et al., 2021; Liu et al., 2022; Susai et al., 2023).