The fibrotic form is typically characterized by irregular linear opacities/coarse reticulations, traction bronchiectasis, and honeycombing, patchy ground-glass attenuation, centrilobular nodules, and mosaic attenuation. Three-density pattern (head cheese sign) is radiological sign which shows a region of the lung with three or more different types of attenuation which can be typical for the fibrotic type. Histopathology
The acute form can be characterized by poorly formed noncaseating interstitial granulomas and mononuclear cell infiltration in a peribronchial distribution with prominent giant cells. The subacute, or intermittent, form produces more well-formed noncaseating granulomas, bronchiolitis with or without organizing pneumonia, and interstitial fibrosis. Much like the pathogenesis of idiopathic pulmonary fibrosis (IPF), chronic HP is related to increased expression of Fas antigen and Fas ligand, leading to increased epithelial apoptosis activation in the alveoli. Cholesterol clefts or asteroid bodies are present within or outside granulomas. Pulmonary Function Testing
Pulmonary function tests (PFTs) can generally reveal a restrictive pattern  however, either a restrictive or obstructive pattern (or both) may emerge on PFTs. PFTs, therefore, are less useful in making a diagnosis but rather tracking improvement or deterioration in lung function following removal or addition of suspected antigens. They may also demonstrate reduced diffusion capacity of lungs for carbon monoxide (DLCO). Bronchoscopy
Bronchoalveolar lavage (BAL) is a reliable way to detect inflammation in the lung airways. Fluid analysis from the lavage extracted from the airways on bronchoscopy often reveals a total elevation in cell count in addition to an elevation in the percentage of T lymphocytes.
Hypersensitivity pneumonitis (HP) or extrinsic allergic alveolitis (EAA) is a syndrome caused by the repetitive inhalation of antigens from the environment in susceptible or sensitized people. Common antigens include molds, bacteria, bird droppings, bird feathers, agricultural dusts, bioaerosols and chemicals from paints or plastics. People affected by this type of lung inflammation (pneumonitis) are commonly exposed to the antigens by their occupations, hobbies, the environment and animals. The inhaled antigens produce a hypersensitivity immune reaction causing inflammation of the airspaces (alveoli) and small airways (bronchioles) within the lung. Hypersensitivity pneumonitis may eventually lead to interstitial lung disease. Signs and symptoms
Hypersensitivity pneumonitis (HP) can be categorized as acute, subacute, and chronic based on the duration of the illness. Acute
In the acute form of HP dose of antigen exposure tends to be very high but only for a short duration. Symptoms may develop 4–6 hours following heavy exposure to the provoking antigen. Symptoms include fever, chills, malaise, cough, chest tightness, dyspnea, rash, swelling and headache. Symptoms resolve within 12 hours to several days upon cessation of exposure. Subacute
Patients with subacute HP gradually develop a productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to the acute form of the disease, but are less severe and last longer. Findings may be present in patients who have experienced repeated acute attacks. Chronic
In chronic HP, dose of the antigen tends to be low volume but for a longer duration. Patients often lack a history of acute episodes.