This means that the ionisation state of chemicals in the gastrointestinal tract and consequently the possibility for passage of chemicals into the body will be different in the infant compared with older children. The volume of distribution for the chemical in a premature infant may be twice as large as than of an adult. The infant only reaches adult levels of most enzyme systems by 2-3 months of age. A low capacity to metabolise xenobiotics may thus make the infant less able to detoxify chemicals and consequently more susceptible to toxicants. The renal excretion depends on the maturation of the kidney.
Tubular secretion is also lower in infants than in adults.
This is counteracted by the low ability to concentrate urine which only reaches adult value at 16 months. Epidemiological data show that the development of allergy from oral sensitisation apparently occurs only within the first year of life. The special sensitivity of infants may be related both to an increased uptake of allergens, and immaturity of the local and systemic immunological responses. Brain weight at birth is about one third of adult weight, and 75 per cent after two years. The blood brain barrier is not complete until around 6 months after birth.