Specific defense factors in breast milk together with length of breast-feeding and genetic predisposition may modulate the development of allergy. We studied whether IgA, soluble CD14 (sCD14), or transforming growth factor (TGF)- in colostrum could affect the development of atopy in children up to age 4. From a cohort of 4676, we selected four groups of children with either long or short exclusive breast-feeding (Ͼ3.5 or Ͻ0.5 mo); these groups further differed in the presence or absence of atopic heredity. In colostrum from mothers, we measured total IgA, IgA antibodies to cow's milk (CM) and casein, sCD14, and TGF-1 and -2. The children were divided into three groups: those with no atopic symptoms or IgE, those with allergic symptoms, and those with both outcomes. Mothers of infants later showing atopic symptoms or, in addition, having IgE sensitization (verified atopy) had a lower concentration of IgA casein antibodies in their colostrum than did mothers of infants with no indication of atopy at age 4. Low concentration of IgA casein antibodies was a significant risk for verified atopy. sCD14 levels were lower in colostrum of mothers with infants developing atopic symptoms and IgE sensitization than of those of infants with no atopy. Specific IgA antibodies to CM antigens and sCD14 in colostrum significantly associated with the appearance of both symptomatic and verified atopy by age 4. Interaction between early infant feeding and subsequent development of allergies has been debated for decades (1-5). Human breast milk contains antibodies to antigens the mother has come into contact with, particularly against food antigens and enteral microbes (6 -10). These antibodies, together with a small amount of food antigens in the milk, may direct the immune response of the infant to develop tolerance or hypersensitivity to those antigens. Breast milk also contains a number of nonspecific anti-infectious substances such as iron-binding lactoferrin, bacteriocidic lysozyme, and the oligosaccharides inhibiting microbial attachment to epithelial cells. It therefore effectively reduces infections of the newborn infant and has an impact on infants' indigenous bacterial flora (8 -10). Human milk, furthermore, contains a number of cytokines (11-16) and other immunomodulatory (17) agents as well as living cells (18), which in the immature intestine of the newborn may modulate the development of local immune responses.We recently showed that the effect of breast-feeding on the appearance of allergic diseases at age 4 y depended on heredity: if the infant has the genetic component for allergy, long breast-feeding seems to protect against allergies; if the heredity is negative, long breast-feeding promotes emergence of allergies (5). Because of such interactions and the fact that maternal heredity has a stronger impact on the appearance of allergies than does paternal allergy (19), breast milk of mothers with allergy has been speculated to differ from that of nonallergic mothers and, in fact, some differences have emerged (20,2...