The metabolic pattern of other people may cause them to react very differently to similar substances. Exposure to drugs and environmental chemicals can decrease liver function, causing a chemical overload in the body and the destruction of enzyme systems. Illness or infection can bring on allergies. Changes take place biochemically in the body during prolonged illness, leading to decreased resistance to allergens. Daily overeating of a specific food can stress enzyme systems so much that they may not be able to handle that particular food efficiently any more.
Stress is a major factor in allergies.
Any stress that is beyond the person's ability to cope can lead to weakness in the organs and immune system. Weakening these systems leaves the person more susceptible to allergies. Allergies are the result of the cumulative effect of many insults to the body over time that have overwhelmed the immune system. The substance itself is not the cause, but the trigger of the allergic reaction. The immune system is the body's main line of defense against foreign invaders.