​Pathology - Hypersensitivity Pneumonitis

​Pathology - Hypersensitivity pneumonitis
The condition is a result of sensitivity to certain antigens, such as actinomycetes commonly present in hay, animal proteins, wood bark, and some chemicals.
Pathophysiology: IgG antibodies bind to the antigen, forming an antibody-antigen complex. This complex triggers an inflammatory response in the alveoli, leading to the release of cytokines and prostaglandins. These substances then attract lymphocytes and macrophages to the lung tissue.

Lung: The acute form is characterized by the presence of mononuclear cells infiltrating the tissue, along with large cells and poorly developed granulomas that do not contain caseous material. These granulomas are mainly found around the bronchial tubes. The chronic type exhibits peripheral irregular fibrosis, especially in the peribronchial area, accompanied with remaining granulomas.

The acute manifestation emerges within a timeframe of 4-12 hours following exposure and encompasses symptoms such as fever, cough, malaise, and dyspnea.
The chronic manifestation is characterized by a gradual onset of difficulty in breathing, gradual loss of weight, and persistent coughing.
Imaging reveals the presence of interstitial fibrosis and ground-glass opacities on CT scans, primarily affecting the upper lobes.
Pulmonary function test results indicate reduced total lung capacity (TLC) and forced capacity (FC), as well as decreased diffusing capacity of the lungs for carbon monoxide (DLCO).

Treatment involves eliminating the causative factor and administering corticosteroids for chronic cases.
Farmer's lung and Bird Fancier's lung are both types of hypersensitivity pneumonitis that result from being exposed to actinomyces (which is present in rotting hay) and avian antigen, respectively.