Hypersensitivity Pneumonitis
- Hypersensitivity pneumonitis (HP) is your lungs throwing an allergic tantrum at something you keep breathing in — bird feathers, moldy hay, hot-tub mist.
- The disease lives in the small airways and surrounding lung, so the imaging hallmarks are diffuse ground-glass, ill-defined centrilobular nodules, and air trapping — a patchy "mosaic" of light and dark lung.
- The signature finding is the three-density / headcheese sign: ground-glass, normal lung, and trapped air all sitting side by side on the same slice.
- Fibrotic HP scars the lung and can look almost exactly like UIP — except the fibrosis tends to spare the very bottom and brings air trapping along for the ride.
- It is usually a mid-to-upper lung disease, which is a handy way to tell it apart from the basal-loving idiopathic fibroses.
Here is a disease that is, at its heart, a grudge. Your lungs meet some harmless little particle — a sprinkle of mold, a fleck of bird dander, a microbe living in your humidifier — and instead of shrugging it off, they decide this is the hill to die on. Every time you breathe the offender back in, the immune system relitigates the whole argument, and the lung tissue takes the damage. That is hypersensitivity pneumonitis (HP), once charmingly called extrinsic allergic alveolitis.
The key to reading it on imaging is remembering where the fight happens: in the smallest airways and the lung immediately around them. Hold onto that and the pictures make sense.
Why it lives in the small airways
The particles that trigger HP are tiny enough to ride the air all the way down to the centrilobular region — the dead center of each little lung unit, where the smallest bronchiole and its artery enter. That is exactly where the inflammation sets up shop. So when you look at the CT, the abnormalities cluster around those central dots, not out at the edges.
This gives you the first classic finding: faint, fuzzy centrilobular ground-glass nodules, sprinkled diffusely like someone took an airbrush to the lung. They are ill-defined — soft cotton-ball edges, not crisp pebbles. If you came here from sarcoidosis, this is the opposite habit: sarcoid loves the lymphatics out along the fissures and bronchovascular bundles; HP loves the lobular center.
The signature: three densities on one slice
Because HP also inflames and narrows those small airways, air gets trapped behind them — it can sneak in but struggles to get back out. On expiratory CT, trapped lung stays stubbornly dark while normal lung empties and turns gray. The result is a patchwork of brightness across the lung field.
Now layer the findings together: some regions are hazy/gray (ground-glass), some are normal, and some are abnormally dark (air trapping). All three living on the same image is the three-density sign — known in radiology's finest culinary tradition as the headcheese sign, because it looks like that marbled deli loaf of assorted bits suspended in jelly. (Radiologists name things after lunch. I don't make the rules.)
The headcheese sign is fairly specific for HP, but the concept underneath it is the real prize: HP simultaneously fills lung with inflammation (lighter) and blocks airways (darker). A disease that does both at once is unusual, which is exactly why this pattern points the finger.
Getting those expiratory images is why HP is best evaluated with a dedicated high-resolution chest CT protocol that includes expiratory and often prone sequences — without the expiratory phase, the air trapping can hide completely.
When it scars: fibrotic HP
If the offender keeps coming and nobody figures out what it is, HP can transition from "inflamed and reversible" to "scarred and permanent." This is fibrotic HP, and it is one of the great mimics in chest imaging because it can produce traction bronchiectasis, reticulation, and even honeycombing — the same vocabulary as UIP/IPF.
A few tells help you separate them:
| Feature | Fibrotic HP | UIP / IPF |
|---|---|---|
| Vertical zone | Often mid/upper, or no clear gradient | Strongly basal (bottom-heavy) |
| Front-to-back | Can be patchy, relative basal sparing | Subpleural, basal predominant |
| Air trapping / lobules | Present — mosaic, spared secondary lobules | Usually absent |
| Clinical clue | Exposure history (birds, mold, etc.) | None; diagnosis of exclusion |
Don't anchor on honeycombing and call everything UIP. Fibrotic HP that scars from the top down with visible air trapping is a different animal with a different cause — and unlike IPF, HP has a treatment that starts with find the bird and get rid of it. Missing the air trapping can cost the patient an actually-fixable diagnosis.
The history is half the diagnosis
Imaging strongly suggests HP, but it rarely closes the case alone. The unsung hero is the exposure history — the parakeet, the down comforter, the water-damaged basement, the metalworking fluid at the factory. The lung is reacting to something, and finding that something both confirms the diagnosis and provides the cure.
When you see diffuse centrilobular ground-glass nodules plus mosaic air trapping, your report should gently nudge the clinician: ask about birds, mold, and hot tubs. A good radiology read on HP often does more for the patient than any drug — because the single most effective treatment is removing the trigger.
So when you meet a lung full of soft fuzzy dots and that marbled headcheese patchwork — especially up high, especially with air trapping — think less "what rare disease is this" and more "what has this person been breathing." That question is the whole game.