**7. Amyloidosis of the respiratory tract**

Amyloidosis is characterized by pathological misfolding of the amyloid protein and its deposition as fibrils leading to organ dysfunction. Amyloidosis of the respiratory tract can be localized or a part of a systemic picture of dysfunction. Pulmonary amyloidosis is only symptomatic if amyloid deposits are present on the alveoli, causing impaired gas exchange. Amyloid deposition in the lung parenchyma may manifest as nodular deposits or lead to localized lymphomas.

Tissue deposits of misfolded amyloid protein in the form of fibrils characterize systemic amyloidosis. There are 15 different kinds of systemic amyloidoses, defined based on the characteristics of the deposited amyloid protein [63]. The primary kinds of amyloidosis that impact the lungs are systemic AL and localized AL consisting of monoclonal light chains, AA amyloid consisting of apolipoprotein serum amyloid A, and ATTRwt consisting of wild-type transthyretin [64]. Regardless of the type of amyloid fibril that is deposited, all fibrils have the same backbone structure that the Congo red stain binds in order to reveal an apple-green birefringence under polarized light [65]. Tissue biopsy is central to diagnosis of amyloidosis as treatment modalities vary based on the type of amyloid protein that is deposited. Less invasive procedures like abdominal fat biopsy and fine needle biopsy are indicated over more invasive procedures like transbronchial biopsy [66, 67]. Once a biopsy is obtained, it is evaluated using immunohistochemistry. The lungs are a common site of amyloid deposition, although not always symptomatic. There are three main kinds of pulmonary amyloidosis: nodular, diffuse, and tracheobronchial. Systemic amyloidosis is symptomatic, and often as a result of chronic inflammation.
