During the first few weeks of the illness, cells of the immune system attack the coronary arteries and release a special substance (MMP) that is responsible for the coronary artery enlargement. There is a common antibiotic, doxycycline that can specifically block the action of this special substance (MMP). Research done on animals with KD showed that doxycycline was able to block this special substance and prevent enlargement of coronary arteries. Research in adults with enlargement of the main artery in their abdomen also showed that doxycycline may improve the outcome. Based on these studies doxycycline may be a promising therapy for children with KD, who develop enlargement of the coronary arteries.