Delaying solids decreases the risk of food allergies.
It is well documented that prolonged exclusive breastfeeding results in a lower incidence of food allergies (see Allergy References and Risks of Artificial Feeding). From birth until somewhere between four and six months of age, babies possess what is often referred to as an "open gut." This means that the spaces between the cells of the small intestines will readily allow intact macromolecules, including whole proteins and pathogens, to pass directly into the bloodstream.This is great for your breastfed baby as it allows beneficial antibodies in breastmilk to pass more directly into baby's bloodstream, but it also means that large proteins from other foods (which may predispose baby to allergies) and disease-causing pathogens can pass right through, too. During baby's first 4-6 months, while the gut is still "open," antibodies (sIgA) from breastmilk coat baby's digestive tract and provide passive immunity, reducing the likelihood of illness and allergic reactions before gut closure occurs. Baby starts producing these antibodies on his own at around 6 months, and gut closure should have occurred by this time also.