A comparison of the percentage of CD4+/PD-L1+ T lymphocytes in selected types of PAH and the control group showed the existence of a significantly higher percentage of these lymphocytes in the iPAH group compared to the control group (p < 0.001), the CTEPH group (p < 0.01), and the CHD-PAH group (p < 0.01). The gene discussed is CD4; the disease is pulmonary arterial hypertension.