C4A and rheumatoid arthritis: While previous studies have implicated low copy number of C4A (Li et al., 2017; Juptner et al., 2018) in autoimmune diseases, including systematic lupus erythematosus and rheumatoid arthritis, increased C4A was associated with schizophrenia susceptibility, possibly through its role in synaptic pruning (Sekar et al., 2016).