Absorption of protein and protein fragments in the developing intestine: role in immunologic/allergic reactions.

An important adaptation of the gastrointestinal tract to the extrauterine environment is its development of a mucosal barrier against the penetration of proteins and protein fragments. To combat the potential danger of invasion across the mucosal barrier the newborn infant must develop within the lumen and on the luminal mucosal surface an elaborate system of defense mechanisms which act to control and maintain the epithelium as an impermeable barrier to the uptake of macromolecular antigens. As a result of a delay in the maturation of the mucosal barrier, newborn infants are particularly vulnerable to pathologic penetration by harmful intraluminal substances. The consequences of altered defense are susceptibility to infection and the potential for hypersensitivity reactions and the formation of immune complexes. With these reactions comes the potential for developing life-threatening diseases such as necrotizing enterocolitis, sepsis, and hepatitis. Fortunately, "nature" has provided a means for passively protecting the "vulnerable" newborn against the dangers of a deficient intestinal defense system, namely human milk. It is now increasingly apparent that human milk contains not only antibodies and viable leukocytes but many other substances that can interfere with bacterial colonization and prevent antigen penetration.