Microscopic polyangiitis is an autoimmune disease characterized by a systemic, pauci-immune, necrotizing, small-vessel vasculitis without clinical or pathological evidence of granulomatous inflammation.

[3] While the mechanism of the disease has yet to be fully elucidated, the leading hypothesis is that AAV (ANCA Associated Vasculitis) develops in patients with a genetic predisposition when an unknown cause triggers the production of p-ANCA.

The neutrophils bind to p-ANCAs and subsequently release inflammatory cytokines, reactive oxygen species and lytic enzymes that cause endothelial injury resulting to inflammation and necrosis of the small vessels.

An important diagnostic test is the presence of perinuclear antineutrophil cytoplasmic antibodies (p-ANCA) with myeloperoxidase specificity[6] (a constituent of neutrophil granules) Depending on which organ is affected special tests can be performed, such as renal biopsy in patients with kidney failure or electromyography in patients with peripheral neuropathy [7] The signs and symptoms of microscopic polyangiitis may resemble those of granulomatosis with polyangiitis (GPA) (another form of small-vessel vasculitis) but typically lacks the significant upper respiratory tract involvement (e.g., sinusitis) frequently seen in people affected by GPA.

[citation needed] Immunsuppressive treatment is the gold standard management in order to achieve remission of the blood vessel inflammation that occurs in active microscopic polyangitis.