Hypersensitivity vasculitis, which is usually represented histopathologically as leukocytoclastic vasculitis (LCV), is a term commonly used to denote a small-vessel vasculitis. There are many potential causes of hypersensitivity vasculitis; however, up to 50% of cases are idiopathic.
Hypersensitivity vasculitis may present clinically as cutaneous disease only or it may be a cutaneous manifestation of systemic disease. The internal organs most commonly affected in hypersensitivity vasculitis are the joints, gastrointestinal tract, and kidneys. Hypersensitivity vasculitis may be acute and self-limited, recurrent, or chronic. Overall, hypersensitivity vasculitis has a favorable prognosis, particularly when no internal involvement is present. Note the image below.
Hypersensitivity vasculitis is thought to be mediated by immune complex deposition.  In this form of vasculitis, circulating antigens in the body (produced by factors such as medications, infections, and neoplasms) induce antibody formation. These antibodies bind to the circulating antigen and create immune complexes, which then deposit within vessels, activating complement and inducing inflammatory mediators. Inflammatory mediators, adhesion molecules, and local factors may affect the endothelial cells and play a role in the manifestations of this disease. 
Additionally, autoantibodies, such as antineutrophil cytoplasmic antibody (ANCA), may be associated with disease manifestations. In ANCA-mediated vasculitis, intracellular proteins from neutrophils become expressed on the cell surface, leading to formation of antibodies (ANCA). These autoantibodies then bind neutrophils, subsequently leading to neutrophil adhesion to vessel walls and cellular activation.
Overall, however, the exact mechanisms causing hypersensitivity vasculitis remain to be elucidated.
The incidence of hypersensitivity vasculitis is unknown, but the condition is presumed to be relatively rare. A 2014 population-based study in Minnesota found the incidence of cutaneous leukocytoclastic vasculitis in adults (including IgA vasculitis as well as other types of small-vessel vasculitis) to be at 45 cases per million. 
Several studies from Spain have been conducted on hypersensitivity vasculitis. [4, 5, 6, 7, 8] Hypersensitivity vasculitis reportedly has an incidence of 10-30 cases per million people per year. Henoch-Schönlein purpura (HSP), a small-vessel vasculitis characterized by deposition of immunoglobulin A (IgA) immune complexes, reportedly has an incidence of 14 cases per million people per year.
Hypersensitivity vasculitis is reported most often in the white population, but epidemiologic studies are not available to assess whether hypersensitivity vasculitis is associated with any specific ethnic group or skin type.
Although hypersensitivity vasculitis appears to affect men and women in approximately equal numbers, some of the studies from Spain suggest that hypersensitivity vasculitis is slightly more common in men than in women. 
Hypersensitivity vasculitis may occur at any age. In both adults and children, Henoch-Schönlein purpura (HSP) may present in a clinically identical fashion to hypersensitivity vasculitis, and biopsy with direct immunofluorescence is typically needed to distinguish the two. HSP is a specific small-vessel vasculitis associated with the presence of vascular IgA deposition. However, by definition, HSP is a clinically defined entity and some authorities may make this diagnosis even if IgA deposition is absent. The classic tetrad of HSP consists of palpable purpura, arthritis, abdominal pain, and hematuria. HSP typically follows a respiratory infection and is more common in the pediatric population.
The prognosis of patients with cutaneous vasculitis depends on the underlying syndrome and the presence of end-organ dysfunction. Patients with disease that primarily affects the skin and/or the joints have a good prognosis. Some patients develop a relapsing disease course, admixed with symptom-free periods. A subset of patients (< 10%) can develop chronic, unremitting disease with recurrent episodes of painful purpura and even ulceration, which can have a considerable impact on quality of life. [9, 10] There is increased morbidity and the potential for mortality if the kidneys, gastrointestinal tract, lungs, heart, or central nervous system are involved.
Patients with granulomatosis with polyangiitis (Wegener granulomatosis), polyarteritis nodosa, eosinophilic granulomatosis with polyangiitis (Churg-Strauss syndrome), or severe necrotizing vasculitis have increased morbidity and mortality. Treatment with corticosteroids and/or immunosuppressive/cytotoxic agents is potentially a life-saving intervention.
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