Microscopic polyangiitis (MPA) is a systemic autoimmune disease characterized by inflammation of small blood vessels, known as vasculitis. Here are the answers to your questions:

Vasculitis: MPA primarily affects small blood vessels, including the arterioles, capillaries, and venules.

Renal involvement: MPA commonly manifests as rapidly progressive glomerulonephritis, leading to kidney damage and potential kidney failure.

Pulmonary involvement: Lung manifestations such as pulmonary nodules, infiltrates, or hemorrhage can occur in MPA.

Skin involvement: Skin lesions, including purpura (small purple spots caused by bleeding), may be present.

Systemic symptoms: MPA can cause systemic symptoms such as fever, fatigue, weight loss, and muscle and joint pain.

Other organ involvement: In addition to the kidneys and lungs, MPA can affect other organs, including the gastrointestinal tract, heart, nerves, and eyes.

The exact cause of microscopic polyangiitis is unknown. It is considered an autoimmune disease, where the immune system mistakenly attacks the blood vessels. The immune response in MPA involves the production of antineutrophil cytoplasmic antibodies (ANCAs), specifically myeloperoxidase (MPO)-ANCAs, which play a role in the development of the disease.

Yes, in most cases of microscopic polyangiitis, patients test positive for perinuclear antineutrophil cytoplasmic antibodies (P-ANCAs) in their blood. These antibodies are directed against the myeloperoxidase (MPO) enzyme within neutrophils. However, it's important to note that not all cases of MPA are P-ANCA positive, and the presence of P-ANCA alone is not diagnostic but can be supportive of the diagnosis.

Microscopic polyangiitis (MPA) and polyarteritis nodosa (PAN) are both types of vasculitis but have some key differences:

Vessel size: MPA primarily affects small blood vessels, while PAN affects medium-sized blood vessels.

Organ involvement: MPA often involves the kidneys and lungs, whereas PAN typically affects the skin, joints, and gastrointestinal tract.

Hepatitis B association: PAN can be associated with hepatitis B infection, while MPA is usually not associated with specific infections.

Histological findings: The histological pattern of inflammation differs between MPA and PAN, with MPA showing a necrotizing vasculitis pattern affecting small vessels, while PAN shows transmural inflammation in medium-sized vessels.

It's important to consult with a healthcare professional for an accurate diagnosis, as the presentation and severity of vasculitis can vary among individuals. Treatment for microscopic polyangiitis typically involves immunosuppressive medications to control inflammation and manage organ involvement.