Presented by: Sean McGhee, MD
Clinical Associate Professor, Pediatric Immunology
Stanford University Medical Center
April 21, 2011
Your eyes itch, your nose is running, you’re sneezing, and you’re covered in hives. These are just some of the typical reactions in people who have an allergy. For most people, allergies can be annoying and uncomfortable. In some cases an allergic reaction can be fatal.
Allergies are abnormal immune system reactions to substances that are typically harmless to most people. Normally, your immune system is your body’s defense system. In most allergic reactions, however, it is responding to a false alarm—normally inoffensive stimuli like certain foods, bee venom, pet dander, or pollen.
Chain Reaction Response
Though scientists are not quite sure how the body becomes sensitized to a particular allergen, they do understand the process behind the reaction, said Sean McGhee, MD, a clinical associate professor of immunology and allergy, who spoke at a presentation sponsored by the Stanford Health Library.
After initial exposure to an allergen, immune system cells called T cells send out chemical messengers that instruct B cells to make a specific antibody targeted to that allergen. The antibody, IgE, binds to mast cells, which are located near surface tissue (like the nose and throat) and lays dormant until you come back in contact with that specific sensitizer.
“IgE is like a trap throughout the body, just waiting for that one sensitizer. It can remain dormant for years,” said Dr. McGhee. “But once it recognizes that sensitizer, IgE triggers the mast cell to respond to it like it’s a foreign invader. The mast cell is primed and ready to go—it’s designed to be a rapid response system. It’s like a bomb ready to blow.”
The mast cell then releases inflammatory mediators, such as histamine, that can spread throughout the body, causing systemic reactions on the eyes, nose, throat, lungs, skin, or gastrointestinal tract. They can also trigger other conditions like asthma and eczema.
Finding the Trigger
When it comes to finding the trigger, doctors usually know where to start. A true allergic reaction tends to be rapid (it shows up within 30 minutes to one hour after exposure) and consistent. About 95 percent of all allergies come from eight foods:
- Tree nuts
An IgE test involves introducing drops of potential allergens on the back or arm and checking the reaction. (A radioallergosorbent test, or RAST, measures the same response through blood samples.) The test follows a specific protocol that includes controls to measure true positive and true negative reactions, so physicians can determine that the reaction is caused by IgE and not by other types of food sensitivity.
“When you test for any disease it best to have a target or you increase your chance of false positives,” said Dr. McGhee. “If IgE is not the cause of the reaction, then it is not a true allergy and the test becomes moot.”
He emphasized that using organic or free-range food products does not affect the allergic reaction since the immune system is responding to a protein inherent in the food.
Living with Allergies
The first step to dealing with an allergy is to identify the allergen through accurate tests so it can be avoided whenever possible. Allergy sufferers need to read labels for ingredients, and Dr. McGhee recommends meeting with a dietician to maintain a healthy diet despite food restrictions.
If the sensitivity to an allergen is extreme, a child may experience anaphylaxis (or anaphylactic shock)—a sudden, severe reaction that can affect the heart or respiratory tract. Parents and family members need to recognize symptoms and learn how to react effectively by keeping injectable eprinephrine immediately available at all times.
Any allergic condition needs to be monitored regularly, since many children grow out of it. “It’s wise to test every year to see if the IgE level has dropped,” said Dr. McGhee. “Life is easier when you don’t have to track diet so carefully. About 25 percent of children will ultimately outgrow their peanut allergy.”
He added that many people respond well to desensitization, which can elevate the threshold of an allergic reaction. This approach is appropriate for children with life-threatening levels of allergy and needs to be conducted under a doctor’s supervision. “Trials have shown that desensitization can induce a fair amount of tolerance for peanut allergy, but does eliminate sensitivity,” he said. “They can tolerate an accidental exposure but not eat a whole peanut butter sandwich.”
Some children have a predisposition toward developing allergic reactions, a condition called atopy, which tends to make them hypersensitive and susceptible to other conditions like asthma, hay fever, and eczema. Babies are born with atopy and then develop resistance, but some people desensitize more slowly than others. This progression, or atopic march, follows an age pattern: usually eggs can be tolerated first and peanuts are the last to go.
Two-thirds of patients with atopy develop allergic rhinitis later in life and about one-half will develop asthma. The stronger the family history of allergic diseases, the greater likelihood a child will be affected.
Other Kinds of Allergies
Hay fever, or allergic rhinitis, causes symptoms like a runny nose, congestion, sneezing, and sinus pressure. It’s caused by an allergic response to outdoor or indoor allergens, such as pollen, dust mites, or pet dander. Hay fever is common—it affects more than 1 in 5 people.
Rhinitis follows the sequential pattern as food allergies: Contact to an allergen primes the mast cells to react whenever there is another exposure. In the Bay Area, the most common outdoor allergens include pollen from grass, oak, olive and ash, mulberry, and juniper and cedar. Since outdoor allergens are harder to avoid than food, but can be managed with medication and desensitization.
Desensitizing using allergy shots needs to carefully designed and measured, Dr. McGhee said. Exposure is built up over a period of six months, followed by a maintenance dose, and most people can stop after three to five years.
“Allergy shots change how the immune system responds,” he said. “As far as prevention, there is no real answer yet but is an active area of research.”
About the Speaker
Sean McGhee, MD, is a clinical assistant professor in the Department of Pediatrics’ Division of Immunology & Allergy. He received his MD at Stanford University School of Medicine and did his internship, residency, and fellowship at the David Geffen School of Medicine at UCLA. He is Board Certified by the American Board of Allergy and Immunology. Dr. McGhee joined the Stanford faculty in February 2011.
For More Information:
Stanford Health Library can do the searching for you. Send us your medical questions.
About Dr. McGhee
Stanford Division of Immunology & Allergy
Packard Children’s Allergy, Asthma, and Immunology