Hypocomplementemic urticarial vasculitis (HUV) is a rare form of vasculitis characterized by inflammation of the small blood vessels and low levels of complement proteins in the blood. HUV causes recurrent episodes of hives (urticaria) and painful skin lesions that itch or burn. Individuals with HUV may also have systemic, multiorgan involvement, causing arthritic joint pain; pulmonary (lung) disease; ocular (eye) inflammation; kidney inflammation; or various other symptoms. Some scientists refer to the condition as HUV syndrome (HUVS) when it is more severe and there is significant systemic involvement. Other scientists call the condition HUVS in the absence of systemic disease. In some cases, the terms are used as synonyms. There appears to be controversy regarding the nomenclature and classification of HUV and HUVS, and whether they are distinct conditions or represent a continuum of the same disease. Both genetic and environmental factors are thought to play a role in causing HUV. It generally occurs sporadically, but familial cases have been reported. It is thought to develop due to an abnormal immune system response involving specific proteins that work together to fight organisms that cause infections. In some cases HUV may be associated with an underlying infection or systemic diseases such as systemic lupus, Sjögren's syndrome, monoclonal gammopathy, or blood disorders.