Merely inheriting appropriate genes for allergy will not necessarily lead to a person showing symptoms of food allergy. A number of environmental and lifestyle factors will determine which allergies he or she experiences. Some of these factors are discussed below.

Increased Permeability of the Lining of the Digestive Tract—the “Leaky Gut”

Permeability refers to the ease with which the molecules pass from the inside of the digestive tract (the gut lumen) through the dividing walls of cells known as the epithelium. The more permeable the epithelium, the quicker and easier it is for food molecules to pass into blood circulation. A very permeable epithelium allows food allergens to easily contact cells of the immune system, whereas the intact, less permeable lining may exclude the larger molecules. In other words, the epithelium acts as a “sieve”; the size of the “holes” in the sieve will determine the size of the particles that pass through. The larger the holes (the more permeable the epithelium), the larger the food molecules that get into the blood. Once in the blood, allergens encounter immune cells. The size of the food molecules is very important in determining whether the immune system starts to reject them or not and, therefore, whether an allergic reaction will occur. In most cases, the larger the molecules, the more likely it is that the immune system will reject them. An increase in the permeability of the epithelium can be due to a number of causes including the following:


The digestive lining is very permeable in the early months of life and gradually matures over the first three to four years. Infants under the age of six months are particularly vulnerable to food allergies. The reason for this is the way in which the immune system is exposed to the food and how it responds to it. The first encounter of the immune system with a food can lead either to tolerance or to sensitization. This is critical in determining whether the child will be able to eat the food in the future without symptoms, or will be allergic to it. In the vast majority of cases, the immune system is tolerized at the first encounter; that is, it is programmed to accept the food as harmless. From then on the immune system does not react to the food when the child eats it. In the rare cases when sensitization occurs, the immune system is programmed to reject the food. This is allergy, and when the child eats the food in the future, the immune system will automatically reject it until the child “outgrows the allergy.” Outgrowing often involves a decrease in the permeability of the epithelial lining of the digestive tract, so the food molecules can no longer pass through into the blood system and encounter the sensitized immune cells.


An inflammatory reaction in the digestive tract (enteritis) can interfere with the lining of the digestive tract and make it more permeable. Food allergens can then come into contact with cells of the immune system more readily, and there is a greater chance that the allergy will occur. Infection, or some other medical condition in the digestive tract, can result in inflammation.
COMBINED ALLERGIC REACTIONS When more than one allergic reaction takes place at the same time, inflammatory chemicals from each of the reactions can add up and reach a level higher than any single reaction alone. For example, an allergy to inhaled allergens, combined with a food allergy, may result in symptoms, whereas the response to the food alone might not be enough to cause symptoms. This may also explain the observation that certain foods eaten together result in a reaction, while the same foods eaten alone may not.


Anything that promotes food absorption through the lining of the digestive tract can increase the allergic response. Alcohol can increase the speed of uptake from the digestive tract as much as cutting in half the time it normally takes for absorption of certain food components. Therefore, having an alcoholic drink at the same time as eating an allergenic food will cause a dramatic rise in the levels of a food allergen. This may result in allergic symptoms, whereas eating the food alone does not.