Quoting BelleDiabolique:" There actually is a reason why they say not before six months - I don't remember all of it, but in the ... [snip!] ... increase risk of food allergies and intolerance because their bodies treat it like a foreign substance rather than nutrition. "
Until around six months of age babies have what is often referred to as an “open gut”. This means the spaces between the cells of the small intestine will allow large molecules to pass directly into their bloodstream. Consequently, as well as the beneficial antibodies in breastmilk passing into baby's bloodstream, allergy-triggering proteins from other foods,