The Myth: Some babies are allergic to their mother's milk.
The Facts: Human milk is the most natural physiological substance that baby can ingest which is why breast milk can be used as a "clear liquid" if a baby is ill with a stomach virus and it is also why breast milk does not sting when placed in the eye as a remedy for conjunctivitis.
If a baby shows sensitivities related to feeding, it is usually a foreign protein that has been consumed by the mother and entered into mother's milk, and not the breast milk. The remedy for this is to remove the offending food from mother's diet for 3-4 weeks. One example is dairy. Dairy is difficult to digest and can take 10 days to two weeks to leave the mother's system and another 10 days to two weeks to leave the nursing baby's system making the process take 3-4 weeks. This lapse in time is no reason to wean because as any formula feeding mother can attest, it may take much longer than that to find a formula your baby can tolerate.
Here are some references to help bust the breastmilk allergy myth!
Hudson, I. et al. A low allergen diet is a significant intervention in infantile colic: results of a community-based study. J Allergy Clin Immunol 1995; 96:886-92.
Mohrbacher, N., Stock, J. BREASTFEEDING ANSWER BOOK. Schaumburg, Illinois: LLLI, 1997; 97-99.
Salmon, M. Breast Milk: Nature's Perfect Formula. Demarest, New Jersey: Techkits, 1994; 32-3.
Newman, Jack: Pitman, Teresa, The Ultimate Breastfeeding Book of Answers, 2003; 150