Alternatively, it is possible that in patients with IPAH the proportions of circulating autoreactive naive B cells is increased as a result of defective self-tolerance in developing B cells, as was also shown in the autoimmune disease systemic lupus erythematosus.31 In any case, we found that 6 out of 11 patients with IPAH had circulating autoantibodies as well as high BTK levels in their peripheral blood B cells. Here, BTK is linked to autoimmune disease.