Any of the interstitial lung diseases in Table 1 can lead eventually to fibrosis, or scarring, of the lung. Therefore, interstitial pulmonary fibrosis (not idiopathic pulmonary fibrosis, or IPF) may be caused from industrial exposure to substances which damage lung tissue, such as asbestos or silica exposure in foundries. Certain drugs used to fight cancer can also cause fibrosis of the lung. However, in idiopathic pulmonary fibrosis, the initial cause of the fibrosis is unknown by definition.

It is suspected that in IPF the lung is exposed to something that causes irritation. The body’s immune system then tries to fight off this irritation with white blood cells, also known as inflammation. This is the same response the body has to infection, for example, where white blood cells kill bacteria by releasing powerful substances. In that way, the body is helped by inflammation. However, white blood cells may also damage normal tissue nearby. This then results in fibrosis or scarring.

In IPF this inflammation occurs in the interstitium and appears to go unchecked for years. The interstitium is the scientific term for the tissue which lines and supports all of the very tiny air sacs which make up the lung (Fig. 1). It is through the walls of these small air sacs in the lung that oxygen diffuses into the blood and waste products like carbon dioxide diffuse out of the blood. When the interstitium becomes scarred and thickened due to inflammation, it is much more difficult for oxygen to diffuse across it from the air sac into the bloodstream. People with interstitial fibrosis, then, will develop symptoms due to this scarring such as shortness of breath and cough.

Viral, immunologic and genetic factors all appear to play an important role in starting the chronic inflammation which leads to fibrosis. Patients will often describe a flu-like illness at the start of their disease. Epstein-Barr virus, influenza virus and hepatitis C virus have all been implicated in some cases. However, to date there is no convincing evidence of prior or persistent viral exposure. Inflammatory and immune processes play a central role in the development of IPF. The inflammation is abnormal in the amount of destruction it causes to normal nearby lung tissue and in its persistence over months and years. Studies have shown that the amount of inflammation is a key factor in determining the prognosis and the response to treatment. However, despite the importance of inflammation, it is the fibrosis resulting from the inflammation that irreversibly destroys the lung and causes symptoms. Finally, a genetic basis for IPF is supported by cases where the disease runs in families. Families have also been identified in which relatives of those with IPF were found to have inflammation in the lung even though they had no symptoms. Therefore, in addition to being exposed to some irritant, it may be that a person must first be genetically susceptible in order to develop IPF. However, no specific genes have been identified so far.

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