Fibrinoid

Fibrinoid refers to an amorphous, eosinophilic, proteinaceous deposit that can accumulate in tissue or the walls of blood vessels during certain immune-mediated or hypertensive injuries. It is not a single substance but a deposition that contains fibrin, immune complexes, and plasma proteins, giving it a bright pink appearance on hematoxylin-eosin stained sections.

Fibrinoid necrosis is a related phenomenon in which the deposited fibrinoid material is associated with cell death and fragmentation of tissue. It most commonly occurs in small- to medium-sized arteries and arterioles affected by immune complex–mediated vasculitis or malignant hypertension.

Pathogenesis involves endothelial injury with leakage of plasma proteins and fibrin into the vessel wall, followed by deposition of immune complexes and complement activation. Overlying necrosis of smooth muscle and connective tissue can ensue, producing a thickened, pale, eosinophilic appearance in the affected area.

Clinical significance: the presence of fibrinoid change supports a diagnosis of vasculitis or hypertensive vascular injury and can influence prognosis and management. It is seen in conditions such as polyarteritis nodosa, hypersensitivity vasculitis, systemic lupus erythematosus, and severe malignant hypertension.

Diagnosis relies on histopathology: routine light microscopy reveals eosinophilic, amorphous material in vessel walls or tissue; immunofluorescence or immunohistochemistry may detect immune complexes or fibrin.