Fibrinoïde

Fibrinoïde, also called fibrinoid material, is a histopathological term describing an amorphous, eosinophilic, proteinaceous deposition that occurs in tissues, most notably within vessel walls, as a result of severe vascular injury. It is commonly discussed in relation to fibrinoid necrosis of vessels, where the material is bright pink on standard staining.

The material comprises a mixture of fibrin, immune complexes, complement fragments, and various plasma proteins. It reflects leakage of plasma contents across damaged endothelium rather than a normal coagulation process. In vessels, the deposition can be confined to the intima and media or extend into the perivascular tissue, where it appears as a dense, homogeneous, eosinophilic zone.

Fibrinoïde changes appear in several pathological contexts. They are characteristic of immune complex–mediated vasculitis such as polyarteritis nodosa and certain hypersensitivity vasculitides, and they can also be seen in malignant hypertension and placental changes associated with preeclampsia/eclampsia. In severe cases, the deposition is accompanied by necrosis of the vascular wall.

Pathogenesis involves endothelial injury leading to plasma protein leakage and immune complex–driven inflammation, with subsequent deposition of fibrin and other proteins. Histologically, the hallmark is a dense, amorphous band within the vessel wall or adjacent tissue, often prompted by necrotizing processes.

Clinical significance and differential: The presence of fibrinoïde material indicates substantial vascular injury and supports a diagnosis of vasculitis or hypertensive-related changes on biopsy. It must be distinguished from true fibrin deposition in coagulation, as well as from amyloid and other nonfibrinoid processes present in tissues. Management focuses on treating the underlying disease.