Some individuals, including some infants, react adversely to cow’s milk protein. This is believed to be a rare, but important, source of crying in early infancy. It is assumed that the infants cry because of gastrointestinal pain, but the mechanisms are not known. The term ‘intolerance’ is preferred, since many such infants do not show evidence of allergic reactions and may ‘grow out’ of their intolerance.
See Colic, Crying, Failure to thrive, Lactose intolerance