An allergy is an abnormal reaction by a person's immune system to a normally harmless substance. A person without allergies would have no reaction to this substance, but when a person who is allergic encounters the trigger, the body reacts by releasing chemicals which cause allergy symptoms.

In children, allergic disease first occurs as atopic dermatitis (eczema) or food allergies. Children with atopic dermatitis are then at an increased risk of developing allergic rhinitis and asthma; both are more likely to occur in school-age children.

Typically, atopic dermatitis goes away by adulthood, as do many types of food allergies.

Allergic rhinitis and asthma, however, most often start during the adolescent, teenage and young adult years, and are likely to persist throughout a person's life. The severity of allergic symptoms, however, may wax and wane, and even temporarily disappear during a person's life.

Atopic Dermatitis

This is typically the first sign of allergies and is seen in 10 to 20% of all children, frequently during infancy. Atopic dermatitis, or eczema, is characterized by itching, with rash formation at the sites of scratching. The rash is typically red and dry, may have small blisters, and can flake and ooze over time.

In infants and very young children, this rash involves the face (especially the cheeks), chest and trunk, back of the scalp and may involve the arms and legs. This distribution reflects where the child is able to scratch, and therefore usually spares the diaper area. The location of the rash changes in older children and adults to classically involve the skin in front of the elbows and behind the knees. Food and environmental allergies have been shown to worsen atopic dermatitis.

Food Allergies

Food allergies can occur at any age. Almost all people with food allergies will have a skin symptom, such as hives, swelling, itching or redness of the skin, as a result of eating the culprit food. These symptoms typically occur within a few minutes of eating the food in question, although they can be delayed up to a couple of hours.

Other symptoms of food allergies can include nausea, vomiting, stomach aches, diarrhea, breathing difficulties (asthma symptoms), runny nose, sneezing, and lightheadedness. In some cases, children can experience a severe allergic reaction, called anaphylaxis, which can be life-threatening.

Nasal Allergies

Allergic rhinitis occurs in up to 30% of adults and up to 40% of children. Symptoms of allergic rhinitis include sneezing, runny nose, itchy nose and eyes and nasal congestion. Some people may also experience post-nasal drip, allergic shiners (dark circles under the eyes), and a line across the nasal bridge from an upward rubbing of the palm of the hand on the nose, a sign called the "allergic salute."

Asthma

Allergies are a major cause of asthma, a condition that occurs in about 8% of all people. Though it can occur at any age, it is most often seen in males in the pre-teen years and in females in the teenage years; asthma is the most common chronic disease in children and young adults. Sometimes asthma is difficult to diagnose in very young children, and may require a physician who is an asthma specialist.

Symptoms of asthma may include:

- Coughing -- This can be the only symptom in some people who have "cough-variant asthma." The cough is often dry, hacking, and may be worse with allergic triggers and after exercise. The cough may only be present at night. Cold air may also trigger this symptom.

- Wheezing -- This is a high-pitched, musical-like sound that can occur with breathing in and out in people with asthma. Wheezing usually occurs along with other asthma symptoms, may get worse with exercise and with allergic triggers.

- Shortness of breath -- Most people with asthma feel as if they're not getting enough air at times, particularly when they are physically exerting themselves or when an allergic trigger is present. People with more severe asthma have shortness of breath at rest or wake-up with this symptom during the night.

- Chest tightness -- Some people describe this as a sensation that someone is squeezing or hugging them. Children may say that their chest hurts or feels "funny."

Many asthmatics have symptoms with exercise; this does not necessarily mean that their asthma is severe or uncontrolled.