Overexpression of B cell-activating factor (BAFF) system molecules has been detected in patients with various types of autoimmune diseases, such as systemic lupus erythematosus (SLE), Sjögren’s syndrome (SS), systemic sclerosis (SSc), bullous pemphigoid (BP), pemphigus vulgaris (PV), and alopecia areata (AA) (Figure 3). This evidence concerns the gene TNFSF13B and acquired polycythemia vera.