A review of the current knowledge of mucosal immune response development indicates that migration of immunocytes to mucosal surfaces is an early event during fetal life. The morphologic development of gut-associated lymphoid tissues (lymphoid folicles, Peyer's patches and tonsils) occurs from 11- 20 weeks' gestation upwards. But they are active only after birth, when usually antigenic challenge takes place. The complete development of secretory immunity is a postnatal achievement. In infancy, some environmental factors such as feeding and oral antigenic challenge seems to modulate immune response development. Breastfeeding, by means of antiinfectious, antiinflammatory and immunomodulating properties, besides completing infant secretory immunity, also stimulates immunological maturation on mucosal surfaces.