3.14 Relation of infectious diseases within the first year of life, development of immune response, and risk of atopy within pre-school age: a prospective birth cohort study

Former epidemiologic studies have consistently shown an inverse relation between number of siblings and allergic diseases. Strachan formulated the hygiene hypothesis, suggesting that the risk of allergic disease is reduced by infectious diseases in infancy transmitted by older siblings (1). Children are born with a tendency to show allergic-type (type 2 cytokine) immune reactions, a pattern that disappears in the first years of life. However, this maturation might be delayed if infants are predisposed to allergy (e.g. by parental history) and in children with reduced exposure to lipopolysaccharides (the latter resulting mainly from exposure to gram-negative bacteria). The dominant Th2 immunity may then cause a higher risk for allergic reactions.