The hygiene hypothesis was proposed to explain the marked increase in allergies that has been observed in industrialized (Westernized) societies. This hypothesis proposes that early and frequent exposure to bacterial and other antigens, such as is common in developing nations, leads to a normal Th1 response, but that better public hygiene and less infections observed in industrialized nations may lead to persistence of the Th2 phenotype and thereby increase our risk for developing allergies. Infection early in life with measles or hepatitis A virus, immunization with bacille Calmette-Guérin, certain gastrointestinal bacteria (lactobacillus), and environmental endotoxin exposure may protect individuals from developing allergy in adulthood. Paradoxically, infestation by parasites stimulates a Th2-cell response; however, the incidence of allergic disease is very low, perhaps due to the stimulation of T-regulatory lymphocytes that can downregulate Th1 and Th2 responses. Some types of human glomerulonephritis (GN) have Th1-predominant immune responses, including crescentic and membranoproliferative GN, whereas other types of GN have a predominant Th2 immune response, including membranous nephropathy, minimal change disease, and immunoglobulin A nephropathy. A review of the prevalence of specific GN shows that the higher prevalence of membranoproliferative GN in developing countries and the higher frequency of immunoglobulin A nephropathy and minimal change disease in industrialized countries could be explained by the hygiene hypothesis. We suggest that studies examining Th1/Th2 balance, particularly as it develops in childhood, should be performed to determine if early polarization of the immune response is responsible for the later development of specific forms of GN. © 2005 by the International Society of Nephrology.