A comparison of the percentage of T cells CD8+CD200+ in selected types of PAH and the control group showed the existence of a significantly higher percentage of these lymphocytes in the group of patients with iPAH than in the control group (p < 0.001) and in the group of patients with CTD-PAH (p < 0.01). The gene discussed is CD200; the disease is idiopathic pulmonary arterial hypertension.