Pulmonary hypertension (PH) is a debilitating disease characterized by structural remodeling of the arterial vasculature of the lung leading to increased vascular resistance and increased pulmonary arterial pressures, right ventricular (RV) hypertrophy, heart failure and ultimately, death. Based on the underlying causes of PH, the WHO classification system divides PH patients into 5 groups: (1) Pulmonary Arterial Hypertension (PAH), (2) PH due to left heart disease, (3) PH due to lung disease, (4) Chronic Thromboembolic PH (CTEPH), and (5) PH with unclear and/or multifactorial mechanisms. In pulmonary arterial hypertension (PAH) patients, vascular endothelial cell proliferation along with concurrent formation of new blood vessels (neoangiogenesis), when exuberant, results in the formation of glomeruloid structures in pulmonary arterioles known as the plexiform lesions. These plexiform lesions are typically defined as dynamic networks of vascular channels formed by uncontrolled proliferation of endothelial cells. Both PAH and CTEPH histology displays extensive accumulation of immune cells. IPAH patient lungs and lung biopsy material from CTEPH patients shows compelling evidence for activation of the innate immune system. This includes pathological involvement of macrophages, mast cells (MCs) and neutrophils by production of inflammatory cytokines, recruitment of other immune cells and local inflammation and damage. Lungs of IPAH patients demonstrate increased numbers of dendritic cells (DCs) , acting as a bridge between innate and the adaptive immune system by presentation of antigens to T cells. DCs contribute to increased production of cytokines and chemokines, attracting other inflammatory cells to the site of inflammation. Dysregulated Th17 immunity is another phenomenon in PAH patients, creating a pro-inflammatory auto-immune environment. Moreover, IPAH patients display an increase of circulating autoantibodies specifically targeting endothelial cell surface antigens. Extensive biomarker research reveals that many inflammatory and immune markers correlate with hemodynamics and/or prognosis of PAH and CTEPH patients. Currently, there is still much unknown about the pathological involvement of the immune system in PAH and CTEPH etiology. Novel insights are required, to gain knowledge into novel pathways for therapeutic targets and new treatment options for researchers and clinicians. Therefore, the aims of this thesis were to; (I) further unravel the immunological imbalance in pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) pathogenesis, (II) to find novel biomarkers and therapeutic targets