In patients with PAH, chemokines secreted locally by pulmonary vessels recruit B cells, which recognize self-antigens via B cell receptors and receive co-stimulatory signals from follicular helper T cells (Tfh) (IL-21), Th2 cells (IL-4/IL-6/IL-13), and mast cell cytokines. Here, IL4 is linked to pulmonary arterial hypertension.