A ferruginous body is a histopathologic

Histopathology

Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease...

 finding in interstitial lung disease suggestive of significant asbestos

Asbestos

Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...

 exposure (asbestosis

Asbestosis

Asbestosis is a chronic inflammatory and fibrotic medical condition affecting the parenchymal tissue of the lungs caused by the inhalation and retention of asbestos fibers...

).

They appear as small brown nodules in the septum of the alveolus. Ferruginous bodies are typically indicative of asbestos

Asbestos

Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...

 inhalation (when the presence of asbestos is verified they are called "asbestos bodies"). In this case they are fibers of asbestos coated with an iron-rich material derived from proteins such as ferritin

Ferritin

Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion. The amount of ferritin stored reflects the amount of iron stored. The protein is produced by almost all living organisms, including bacteria, algae and higher plants, and animals...

 and hemosiderin

Hemosiderin

thumb|Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderinHemosiderin or haemosiderin is an iron-storage complex. It is always found within cells and appears to be a complex of ferritin, denatured ferritin and other material...

. Ferruginous bodies are believed to be formed by macrophages that have phagocytized and attempted to digest the fibers.