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Breast milk is important for the infant's digestive system. It is the best substance to give, especially over cow's milk. Infants cannot properly digest fats, which cow's milk is full of. Breast milk contains a lot of fat, too, but it also contains lipase, a substance to help break down the fat to aid in digestion. This leads to infants passing softer stools, so constipation is rare in breastfed infants. Human milk also allows beneficial bacteria to grow in the infant's intestines, which protects the mucosal barrier of the infant's stomach. This prevents harmful pathogens from harming the infant's intestinal lining. The infant's digestive mucosa is unable to produce antibodies until they are about four to six months old, which makes the infant susceptible to many infections. However, breast milk provides the antibodies needed for the infant to stay protected until they are able to produce their own antibodies. Breast milk also stimulates a microbiota, which results in the production of IgA. IgA is an immunoglobulin that is a first line of defense to protect the digestive tract of the infant. This immunoglobulin is much higher in infants that are breastfed than in infants that were infant formula-fed.