Vasculitis: Types and Pathological Mechanisms

Core Idea

Vasculitis is inflammation of blood vessel walls caused by immune-mediated mechanisms (antibodies, immune complexes, T cells) or infectious triggers. Classification by vessel size (large, medium, small) and type of inflammation (granulomatous, necrotizing, lymphocytic) guides diagnosis. Large vessel vasculitis (temporal arteritis, Takayasu's) predominantly affects aorta and major branches. Medium vessel vasculitis (PAN, Kawasaki) affects named arteries. Small vessel vasculitis (ANCA-associated, anti-GBM) affects capillaries and venules, causing necrotizing inflammation. Vessel damage leads to ischemia, infarction, and hemorrhage depending on location.

How It's Best Learned

Use a classification matrix organized by vessel size and type of inflammation. Study the pathogenesis of each major form: ANCA-associated vasculitis (PR3/MPO antibodies), anti-GBM disease (basement membrane antibodies), immune complex deposition in IgA nephropathy.

Common Misconceptions

Not all vasculitis is autoimmune; infections can trigger vasculitis through direct invasion or immune complex formation. Vessel size classification predicts distribution of disease (large vessel vasculitis affects proximal aorta; small vessel vasculitis affects kidneys and lungs).

Explainer

From your study of acute inflammation, you know how the immune system mounts a response when it recognizes danger: vasodilation, neutrophil recruitment, cytokine release, tissue remodeling. In vasculitis, this inflammatory machinery is directed at the wrong target — the blood vessel wall itself. The vessel becomes both the battlefield and the casualty. What makes vasculitis conceptually tractable is that it follows a clear organizing logic: the immunological mechanism determines what drives the inflammation, and the vessel size determines where in the body it manifests.

Three major immunological mechanisms drive vasculitis. First, direct antibody attack: in anti-GBM disease (Goodpasture syndrome), IgG antibodies target collagen IV in the glomerular and alveolar basement membranes, triggering complement activation and neutrophil influx that destroys capillary walls — producing pulmonary hemorrhage and glomerulonephritis simultaneously. Second, immune complex deposition: in IgA vasculitis (Henoch-Schönlein purpura) and lupus vasculitis, antigen-antibody complexes deposit in small vessel walls, activate complement via the classical pathway, and recruit neutrophils that degranulate and damage the endothelium. Third, the most mechanistically distinctive: ANCA-mediated neutrophil activation. Anti-neutrophil cytoplasmic antibodies (ANCA) target intracellular enzymes — proteinase-3 (PR3) or myeloperoxidase (MPO) — that are normally sequestered inside neutrophil granules. When neutrophils are primed by cytokines (as in infection or systemic inflammation), they transiently express these antigens on their surface; ANCAs bind them, activating the neutrophil against the endothelium of small vessels. The result is necrotizing inflammation of arterioles and venules without immune complex deposition — a "pauci-immune" vasculitis.

Vessel size predicts the clinical syndrome with remarkable precision. Large vessel vasculitis (temporal/giant cell arteritis, Takayasu's arteritis) involves the aorta and its major branches. Granulomatous inflammation infiltrates the vessel wall, destroying the media and leading to aneurysm or stenosis. Giant cell arteritis in the temporal artery causes jaw claudication and headache; in the ophthalmic artery, it causes sudden blindness — a medical emergency. Takayasu's affects the aortic arch branches, causing arm claudication and absent pulses in young women. Medium vessel vasculitis (polyarteritis nodosa, Kawasaki disease) affects named muscular arteries. PAN causes segmental necrotizing inflammation with microaneurysms visible on angiography; renal and mesenteric involvement produces infarction. Kawasaki disease — triggered by an unidentified infectious agent in genetically susceptible children — affects the coronary arteries, and untreated, coronary artery aneurysms rupture or thrombose, causing myocardial infarction in children. Small vessel vasculitis (ANCA-associated vasculitis, IgA vasculitis) strikes capillaries, arterioles, and venules — the vessels supplying the kidney glomeruli, alveolar capillaries, and dermal capillaries. Hallmarks are glomerulonephritis (hematuria, proteinuria, renal failure), pulmonary hemorrhage (hemoptysis), and palpable purpura (skin). A patient with simultaneous pulmonary hemorrhage and glomerulonephritis — "pulmonary-renal syndrome" — should immediately prompt evaluation for small vessel vasculitis.

The organ consequences follow from vessel anatomy: destroy the vessels supplying a structure, and that structure becomes ischemic or infarcted. This is why recognizing the level of the vascular tree involved — felt through clinical presentation and confirmed by biopsy — directs both the differential diagnosis and the treatment. Treating ANCA vasculitis requires immunosuppression targeting neutrophil-endothelial interactions and B-cell-derived antibody production; treating giant cell arteritis requires suppressing the granulomatous T-cell response. The shared diagnosis of "vasculitis" conceals mechanistically distinct diseases that respond to mechanistically targeted therapies.