when an antiserum is given, the human immune system can mistake the proteins present for harmful antigens. the body produces antibodies, which combine with these proteins to form immune complexes. these complexes precipitate, enter the walls of blood vessels, and activate the complement cascade, initiating an inflammatory response and consuming much of the available complement component 3 (c3). the result is a leukocytoclastic vasculitis. this results in hypocomplementemia, a low c3 level in serum. they can also cause more reactions resulting in typical symptoms of serum sickness.