A comparison of the percentages of CD4+/PD-1+ T lymphocytes in the selected types of PAH and the control group revealed the existence of a significantly higher percentage of these lymphocytes in both the iPAH (p < 0.001) and CHD-PAH (p < 0.001) groups than in the control and CTD-PAH groups. This evidence concerns the gene CD4 and idiopathic pulmonary arterial hypertension.