Found this on askdrsears.com......... To prevent potentially-allergenic foods from entering the bloodstream, the maturing intestines secrete IgA , a protein immunoglobulin that acts like a protective paint, coating the intestines and preventing the passage of harmful allergens. In the early months, infant IgA production is low (although there is lots of IgA in human milk), and it is easier for potentially-allergenic food molecules to enter the baby's system. Once food molecules are in the blood, the immune system may produce antibodies to that food, creating a food allergy . By six to seven months of age the intestines are more mature and able to filter out more of the offending allergens. This is why it's particularly important to delay solids if there is a family history of food allergy, and especially to delay the introduction of foods to which other family members are allergic.
at 10:51 AM on May. 31, 2009