ATLAS OF SKIN DISEASES

Vasculitis can be caused by circulating immune complexes,  which activate a mechanism of the small vessel's wall endothelium injury.  Any antigens, such as drugs or infectious agents could be involved in formation of immune complexes. 

Histologic chages include fibrinoid necrosis of walls of small blood vessels and infiltration of leukocytes with fragmentation of nuclei (leukocytoclastic vasculitis with karyorrhexis or leukocytoclasia).