TNFRSF13C and systemic lupus erythematosus: Serum BAFF and soluble BAFF receptor (sBAFF‐R) were quantified using ELISA, and soluble B cell maturation antigen (sBCMA) and transmembrane activator and cyclophilin ligand interactor (sTACI) by Luminex, in 87 SLE patients and 17 healthy controls (HC).