Imunological and clinical characteristics in children with polyarteritis nodosa: a retrospective study over the last 20 years by Malenica Branko, Potocki Kristina, Starcevic Katarina, Vidovic Mandica, Milosevic Danko, Batinic Danica, Malcic Ivan, Topic Masa, Jelusic-Drazic Marija