Fibrinoide

Fibrinoide, or fibrinoid material, is a term used in pathology to describe eosinophilic, fibrin-like deposits found in tissues and especially within the walls of blood vessels. It is not true fibrin, but a histological appearance that results from the accumulation of fibrin, immune complexes, complement, and plasma proteins in areas of vascular injury. When the deposition is extensive and accompanied by cell death, it is referred to as fibrinoid necrosis.

Fibrinoid deposition is a feature of several conditions that involve immune-mediated damage or severe vascular stress. It is commonly seen in certain types of vasculitis, such as polyarteritis nodosa and microscopic polyangiitis, where immune complexes and inflammatory mediators injure vessel walls. It also occurs in hypertensive emergencies, particularly malignant hypertension, where arteriolar walls become thickened and necrotic. In obstetrics, fibrinoid changes can be found in placental and decidual vessels in preeclampsia and HELLP syndrome, contributing to placental insufficiency. In transplanted organs, fibrinoid necrosis can signal rejection or injury.

Diagnosis relies on histopathology. Microscopy reveals bright pink, amorphous material within vessel walls on standard hematoxylin-eosin staining. Special stains and immunopathological studies help distinguish fibrinoid deposits from other substances such as amyloid or collagen. Clinically, fibrinoid changes indicate significant vascular injury and are typically interpreted within the context of the underlying disease process, with prognosis and treatment guided by the associated condition.