In short, three different stages in the development of ILD can be discerned. Firstly, the lung tissue is damaged. In sarcoidosis this damage is thought to be antigen-driven, multisystemically and leading to the formation of granuloma. Moreover, it is suggested that genetic factors play an important role in the genesis of sarcoidosis. In IPF the exact etiology of this damage is unknown, but it has been speculated to be inflicted by an unidentified stimulus that produces repeated episodes of acute lung injury Secondly, the walls of the air sacs in the lung become inflamed as a reaction to the caused damage. This results in the activation of inflammatory cells like macrophages that cause the expression of pro-inflammatory cytokines, especially interleukin-10 and tumour necrosis factor (TNF)-alpha, in the lungs.