It is becoming more widely acknowledged that B lineage cells in EGPA patients display a dysregulation in various receptors and ligands, such as B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL), as well as downstream effector molecules, including Bruton’s tyrosine kinase (BTK) and spleen tyrosine kinase (SYK) (132, 133). The gene discussed is BTK; the disease is eosinophilic granulomatosis with polyangiitis.