Complex lesions 
The pathological classification of the World Symposium meetings in Venice and in Dana Point comprises three patterns, plexiform lesion, dilation lesion and arteritis. The plexiform lesion probably represents the most illustrious form of vascular lesions in PAH and affects various vascular compartments: focal intimal thickening of small pulmonary arteries, preferably beyond branching points and exuberant endothelial cell proliferation, leading to the formation of capillary-like, sinusoidal channels on a smooth muscle cell and collagen-rich matrix within the native arterial lumen and resulting in obstruction [173,174]. This glomeruloid-like arterial zone feeds into dilated, vein-like congestive vessels, which are perceivable at low magnification (Figure 2F). The latter vein-like vessels are also known as dilation lesions and may predominate the histological pattern (Figure 2G). Classical arteritis with transmural inflammation and fibrinoid necrosis, as first described by Heath and Edwards for PAH associated to congenital cardiac disease (Eisenmenger), is not a regular finding in PAH [175]. Nevertheless, perivascular inflammatory infiltrates of diseased pulmonary arteries in PAH-patients, consisting mainly of T- and B-lymphocytes, dendritic cells, mast-cells and macrophages can be regularly found [141,148,176] (Figure 2H). It has not been elucidated until now, whether this inflammatory pattern is of pathogenetic importance, or if it represents a pure epiphenomenon within disease evolution. The reported evidence of proinflammatory mediators, so called chemokines, released by altered endothelial cells of PAH-lungs strongly indicates a self-supporting and self-amplifying process [145,177].