Leukocytoclastic vasculitis of small vessels is capable of damaging vessel walls and causing palpable purpura. It may develop in the context of a hypersensitivity reaction, rheumatoid disease, dysproteinemias, and systemic vasculitis associated with positive antineutrophil cytoplasmic antibodies (ANCAs) (Table 179-1; see also Chapter 146). Immunologically mediated, necrotizing neutrophilic infiltration of arterioles, capillaries, and venules occurs. The process can be limited to the vessels of the skin or involve small- to medium-sized vessels of any organ; renal and pulmonary involvement are common and potentially life threatening in the ANCA-positive systemic vasculitides. The two most common of the ANCA-positive conditions, granulomatosis with polyangiitis (Wegener granulomatosis) and microscopic polyangiitis, have different genetic predispositions involving distinct histocompatibility loci accounting for differences in autoimmunity profiles and immunopathology (see Chapter 146). In rheumatoid disease, the postcapillary venules are the principal site of leukocytoclastic injury; systemic involvement is the rule.

The skin lesions of leukocytoclastic vasculitis typically begin as small macules that become palpable and may turn confluent or nodular. The petechial papules do not blanch and as such are designated palpable purpura. They appear in symmetric fashion and predominate in dependent areas. Urticaria, vesicles,
and necrotic ulcerations may also develop. Fever, arthralgias, myalgias, arthritis, pulmonary infiltrates, effusions, pericarditis, peripheral neuropathy, abdominal pain, bleeding, and encephalopathy can occur along with the petechial rash if systemic involvement is present. The skin commonly itches, stings, or burns. Hematuria and proteinuria indicate renal injury; hemoptysis and pulmonary infiltrates on chest x-ray are manifestations of pulmonary vascular involvement.