IPF: scarring with no known cause
Idiopathic pulmonary fibrosis is the prototype and the most feared. 'Idiopathic' means we cannot find a cause; the lung simply scars, relentlessly, in the UIP pattern. It typically strikes people over 60, more often men and former smokers, and announces itself with a dry cough, exertional breathlessness, basal Velcro crackles, and sometimes finger clubbing. Untreated, IPF tends to progress steadily, though the pace varies a lot between individuals. It is the diagnosis that antifibrotic drugs were designed for, so getting the label right has real consequences.
Sarcoidosis and hypersensitivity pneumonitis
Sarcoidosis is an inflammatory disease that builds tiny clumps of immune cells called granulomas in the lung — and often in lymph nodes, skin, eyes, and elsewhere. We still do not know exactly what triggers it. It favours younger adults, leans toward the upper lungs, and classically enlarges the lymph nodes at the hilum of both lungs. Crucially, many cases are mild or even resolve on their own, and when treatment is needed it is usually granuloma-quieting steroids, not antifibrotics — a completely different path from IPF.
Hypersensitivity pneumonitis (HP) is the lung's allergic over-reaction to something inhaled repeatedly — mould from hay or a damp home, proteins from pet birds, certain chemicals at work. The single most powerful 'treatment' is detective work: find and remove the trigger. In its early, inflammatory phase HP can improve dramatically once the exposure stops. Left unrecognised for years, though, it can scar into a fibrotic, IPF-like picture — which is exactly why the careful history, asking about birds, hot tubs, mould, and jobs, is never a formality.
When ILD is part of a bigger disease
Sometimes the lung scarring is one symptom of a body-wide autoimmune condition — rheumatoid arthritis, scleroderma, and similar disorders. This is connective-tissue-disease ILD, and it is easy to miss if you look only at the chest. The clues are elsewhere: stiff swollen joints, tight skin, a rash, Raynaud's fingers that blanch in the cold, dry eyes and mouth. Catching these lets a doctor treat the underlying immune disease, which often calms the lung too — another reason the same HRCT picture can demand a very different plan depending on its cause.
There is a fifth member worth naming briefly: pneumoconiosis, the ILD caused by inhaled mineral dust over many years — asbestos, silica, coal. The cause is in the occupational history, and prevention is everything. Across all of these, the single thread is that fibrosis looks similar on the surface but is reached by very different roads — and the road determines the treatment.