CD4+ T cells isolated from patients with PAH contain higher levels of IL-17A.38 39 Additionally, the numbers of Th17 cells are increased and these cells are often localised in perivascular regions.6 38 39 Also adoptive transfers of Th17 cells to Rag1−/− mice, lacking mature T cells and B cells, induced PH symptoms independent of chronic hypoxia.38 However, whether the cTfh17 or cTfh17.1 cells represent a functional and clinically relevant T-cell subtype is currently unknown and is a topic for further research. This evidence concerns the gene CD4 and pulmonary arterial hypertension.