History:

Leukocytoclastic vasculitis (LCV), also known as hypersensitivity vasculitis or allergic vasculitis, is a small vessel vasculitis.   Although the exact pathogenesis of LCV remain unknown but it is believed that immune-complex deposition (involving IgG and IgM) along the postcapillary venules triggers the subsequent complement activation and production of leukotactic factors.  Neutrophils migrate and release lysosomal enzymes, causing local destruction of the vessel wall and subsequent fluid leakage and extravasation of red blood cells, which along with inflammation, result in the classic palpable purpura associated with LCV.    

LCV may be localized to the skin, or it may manifest in other organs. The disorder may be acute or chronic.  Palpable purpura is the most sensitive diagnostic sign for LCV.  The nonblanching lesions of  LCV first appear as purpuric macules and tend to symmetrically affect dependent regions, primarily the legs and ankles.  Lesions evolve to 3-6 mm papules with sharp margins which may subsequently evolve to vesicles, nodules, ulcerations, or necrotic lesions.  The palpability of these lesions is secondary to inflammation and is differentiated from the nonpalpable purpura associated with thrombocytopenia, trauma, or weakened or abnormal blood vessels.  Multisystem organ involvement occurs in up to 50% of patients, most commonly in the small vessels of the kidneys, gastrointestinal tract, joints, muscles, lungs, and peripheral nerves. 

Ref:

Arvan ME, Brodell RT. A palpable clue to vasculitis. Postgrad Med. 1999 Mar;105(3):229-32.