What Is an Allergy? Allergies are an overreaction of the body's natural defense system that helps fight infections (immune system). The immune system normally protects the body from viruses and bacteria by producing antibodies to fight them. In an allergic reaction, the immune system starts fighting substances that are usually harmless (such as dust mites, pollen, or a medicine) as though these substances were trying to attack the body. This overreaction can cause a rash, itchy eyes, a runny nose, trouble breathing, nausea, and diarrhea. An allergic reaction may not occur the first time you are exposed to an allergy-producing substance (allergen). For example, the first time you are stung by a bee, you may have only pain and redness from the sting. If you are stung again, you may have hives or trouble breathing. This is caused by the response of the immune system. Most people will have some problem with allergies or allergic reactions at some point in their lives. Allergic reactions can range from mild and annoying to sudden and life-threatening. Most allergic reactions are mild, and home treatment can relieve many of the symptoms. An allergic reaction is more serious when severe allergic reaction (anaphylaxis) occurs, when allergies cause other problems (such as nosebleeds, ear problems, wheezing, or coughing), or when home treatment does not help. Food allergies A food allergy is an abnormal response to a food by your immune system. Normally, the immune system protects your health by defending the body against harmful bacteria and viruses. With a food allergy, the immune system identifies certain foods as harmful and triggers an allergic reaction when you eat them. Food allergies are more common in children than adults. Food allergies are most common in people who have an inherited tendency to develop allergic conditions. These people are more likely to have asthma and other allergies. Cereal allergies may appear in a baby when you begin to add cereal to the baby's diet. Children tend to outgrow many food allergies by age 3. The most common foods involved are cow's milk, eggs, nuts, shellfish, soy products, and wheat. Most people who have allergies to seafood, eggs, peanuts, and tree nuts do not outgrow them. Symptoms of food allergies can range from mild and annoying to severe and life-threatening. Symptoms of food allergies can begin right away or within a few hours and can include: A severe allergic reaction (anaphylaxis), which can occur suddenly and quickly become life-threatening. Anaphylaxis can cause wheezing or difficulty breathing, rapid swelling of the throat or tongue, hives, nausea or vomiting, and faintness. In general, the sooner the reaction begins, the more severe it will be. Call 911or other emergency services immediately if you are having a severe allergic reaction. Swelling and itching of the mouth, tongue, or throat. Skin reactions , such as hives, angioedema, or atopic dermatitis. Respiratory reactions, such as allergic asthma and allergic rhinitis. Digestive system reactions, such as nausea, vomiting, diarrhea, stomach cramps, rectal itching, and colic. Headaches. You may be able to prevent food allergies by changing your diet and not eating the foods that you suspect are causing your symptoms. Do not eat these foods for 2 weeks. Add the foods back to your diet, one item at a time, to determine which food is causing problems. This can be done at home unless severe allergic reactions have occurred in the past, such as difficulty breathing or wheezing, facial swelling, itching of the lips or mouth, or hives. If this is the case, eating suspect foods should be done only in a clinic or hospital setting under direct medical supervision. Talk to your health professional or a dietitian before you remove a food from your diet for more than 2 weeks. An unbalanced diet can be harmful. A dietitian can help you change your diet to make sure you are getting proper nutrition. Reactions to food Some reactions to foods are not caused by allergies. Common causes of food reactions include: Lactose intolerance. This is an inability to digest sugar (lactose) in dairy products because a person's body doesn't have the chemical (enzyme) that breaks down the sugar. Lactose intolerance causes stomach or intestinal cramps and diarrhea. Chinese restaurant syndrome (CRS). Monosodium glutamate (MSG), a seasoning that is often used in Asian cooking, may cause dizziness, sweating, ringing in the ears, and a feeling of faintness in some people shortly after they have eaten foods that contain MSG. Wheat intolerance, such as celiac disease. Children are more likely than adults to have difficulty digesting foods that contain wheat, such as bread, crackers, and cereal. These foods are likely to cause them to develop gas. Food poisoning. Digestive problems (gastrointestinal diseases). Emotional problems or stress. Talk to your health professional about a referral to an allergy specialist if you have food allergies. Seasonal allergies Seasonal allergies show up at the same time of the year every year, if you continue to live in the same part of the country. Hay fever is the most common seasonal allergy. Symptoms of seasonal allergies include: Itchy, watery eyes. Sneezing. Runny, stuffy, or itchy nose. Temporary loss of smell. Headache and fatigue. Dark circles under the eyes ("allergic shiners"). Drainage from the nose down the back of the throat (postnasal drip). Sore throat or coughing. Snoring.