Pathogenetic mechanisms of and host defense mechanisms against mucormycosis. To cause disease, the agents of mucormycosis must scavenge from the host sufficient iron for growth, must evade host phagocytic defense mechanisms, and must access vasculature to disseminate. A) In a normal host, primary defense mechanisms against mucormycosis include sequestration of iron in serum by specialized iron-binding proteins (1), phagocytes including circulating neutrophils (2a) and tissue macrophages (2b), and endothelial cells (3), which regulate vascular tone and permeability. Acting in concert, these mechanisms prevent establishment of infection in tissue and subsequent endovascular invasion. B) In susceptible hosts, normal defense mechanisms break down. For example, in diabetic ketoacidosis (DKA), the acidic pH of the serum causes dissociation of free iron from sequestering proteins (1). This release of free iron allows rapid fungal growth. Defects in phagocytic defense mechanisms (2), for example, deficiency in cell number (neutropenia) or functional defects caused by corticosteroids or the hyperglycemia and acidosis of diabetic ketoacidosis, allow proliferation of the fungus. Finally, adherence to and damage of endothelial cells by the fungus (3) allows fungal angioinvasion and vessel thrombosis and subsequent tissue necrosis and dissemination of the fungal infection.