Vasculitis is inflammation of the blood vessels (arteries, veins, or capillaries), which can cause vessel walls to thicken, weaken, narrow, or become blocked, reducing or stopping blood flow and leading to tissue or organ damage. It encompasses a group of rare autoimmune disorders classified by the size of the affected vessels: large-vessel (e.g., giant cell arteritis or Takayasu arteritis, often involving the aorta and major branches), medium-vessel (e.g., polyarteritis nodosa or Kawasaki disease, affecting muscular arteries), and small-vessel (e.g., ANCA-associated vasculitides like granulomatosis with polyangiitis or microscopic polyangiitis, or immune complex types like IgA vasculitis/Henoch-Schönlein purpura). Symptoms vary widely depending on the vessels and organs involved but commonly include fever, fatigue, weight loss, muscle/joint pain, skin rashes (purpura or nodules), nerve problems (numbness, weakness), kidney issues (blood/protein in urine), lung involvement (cough, shortness of breath), or headaches/visual changes in cranial forms. It can be primary (idiopathic, autoimmune-driven) or secondary (triggered by infections, medications, cancers, or other connective tissue diseases).