In order to prevent the penetration of intraluminal material such as micro-organisms, food antigens, toxins etc. across the intestinal mucosa, a complex defence mechanism has been developed. This mechanism consists of non-immunological defence with enzymatic detoxification as well as an immunological one. The nature of the immunological defence has been thoroughly elucidated during the past 10-15 years. This mechanism is in man based on a special local immunological resistance, where production of secretory IgA plays the main role. Immunoglobulin producing plasma cells appear relatively late after birth in the lamina propria of the gut mucosa. Therefore a physiological immaturity of the immunological defence during first weeks of life can be anticipated. The role of the immunological defence system in the control of dietary antigen penetration across gut epithelium has been much discussed. Circulating antibodies to cow's milk proteins in small infants after milk ingestion and the almost constant finding of such antibodies in high titre in patients with selective IgA deficiency even without intestinal disease indicate a highly significant function of this local immunity in antigen handling. It has been suggested that the apparently high incidence of food allergy in infancy is due to immaturity of this system. Cow's milk protein induced enteropathy is also strictly bound to this age group.