Allergies: Types, Symptoms and Treatment

How allergies work

In an allergy, the immune system mistakes a harmless protein for a threat. On first exposure, it makes IgE antibodies against it. These attach to mast cells in the skin, airways and gut. On the next exposure, the allergen binds the IgE and the mast cells release histamine and other chemicals. That causes the itching, swelling, mucus, sneezing and tightening of airways within minutes.

Allergies run in families. Children of allergic parents are more likely to develop allergies, though not always the same ones. People prone to allergies (atopy) often have several: eczema, food allergy, hay fever and asthma, sometimes appearing in sequence through childhood (the "atopic march").

Types of allergies

Hay fever

Allergic rhinitis affects up to 1 in 4 adults. Seasonal symptoms follow tree pollen in spring, grass in late spring and summer, and ragweed in late summer and fall. Year-round symptoms point to dust mites, pets or mold.

Food allergy

Food allergy causes symptoms within minutes to 2 hours of eating. Nine foods cause most reactions in the US. Milk and egg allergies are often outgrown; peanut, tree nut, fish and shellfish allergies usually last for life. Food intolerance, such as lactose intolerance, is different: it causes digestive symptoms without involving IgE and is not dangerous.

Drug allergy

True penicillin allergy is much less common than people think. About 10 percent of people report it, but fewer than 1 percent actually have it when tested. Testing can safely remove the label. See drug allergy.

Allergy symptoms

• nose: sneezing, itching, clear runny nose, congestion
• eyes: itching, redness, watering, puffy lids
• skin: hives, itching, swelling, eczema flares
• mouth: itching or tingling, lip or tongue swelling
• lungs: cough, wheezing, chest tightness, shortness of breath
• gut: nausea, vomiting, cramps, diarrhea (mainly with food allergy)

Anaphylaxis

Anaphylaxis is a severe reaction that involves two or more body systems, or a sudden drop in blood pressure. Symptoms usually start within minutes: hives, throat tightness, wheezing, vomiting, dizziness and collapse. Epinephrine (adrenaline) is the only first-line treatment. Give it immediately into the outer thigh, call 911, lie the person down with legs raised (or sitting up if breathing is hard), and give a second dose after 5 to 15 minutes if there is no improvement and a second device is available. Antihistamines alone are not enough.

Allergy testing

An allergist chooses tests based on your history:

• Skin prick testing: tiny amounts of allergens are pricked into the skin; a raised itchy bump within 15 minutes suggests sensitization. Stop antihistamines a few days beforehand.
• Blood tests (specific IgE): measure antibodies to particular allergens; useful when skin testing is not possible.
• Component testing can help predict how severe a food allergy is likely to be.
• Oral food challenge: eating gradually increasing amounts of a food under supervision, the gold standard for confirming or ruling out food allergy.
• Drug challenge: used to remove a penicillin allergy label.
• Patch testing for contact allergies.

A positive test alone does not prove an allergy; results must match symptoms. Unproven tests such as IgG food panels, hair analysis and kinesiology are not recommended.

Treatment

Avoiding triggers

• Pollen: keep windows closed in high-count seasons, shower after being outdoors, use HEPA filters.
• Dust mites: allergen-proof bed covers, wash bedding weekly at 54 °C (130 °F) or higher, reduce humidity.
• Pets: keep animals out of the bedroom; wash hands after contact.
• Mold: fix leaks and damp; keep humidity below 50 percent.
• Foods: read labels carefully, including "may contain" warnings, and tell restaurants.

Medicines

• Non-drowsy antihistamines (cetirizine, loratadine, fexofenadine) for sneezing, itching and hives. Follow the label dose.
• Steroid nasal sprays (fluticasone, mometasone, budesonide): the most effective treatment for hay fever; use daily during the season.
• Antihistamine eye drops for eye symptoms.
• Decongestants, short term only; avoid with high blood pressure.
• Montelukast for some people with asthma and allergies; it carries a warning about mood and behavior changes.
• Epinephrine auto-injectors for anyone at risk of anaphylaxis. Carry two at all times.

Immunotherapy

Allergen immunotherapy gradually trains the immune system to tolerate an allergen:

• allergy shots, given over 3 to 5 years, for pollen, dust mites, pets and insect venom; venom immunotherapy is highly effective at preventing sting anaphylaxis
• under-the-tongue tablets for grass, ragweed and dust mite allergy
• oral immunotherapy for peanut allergy in selected children

Biologic injections such as omalizumab can reduce reactions in some people with severe asthma, chronic hives or multiple food allergies.

Preventing allergies in children

Current US guidance recommends introducing peanut-containing foods (in a safe form, such as thinned peanut butter) and other common allergens to infants around 4 to 6 months, once they are ready for solids. Babies with severe eczema or egg allergy should be assessed by their pediatrician first. Early introduction reduces the risk of developing peanut allergy. Never give whole nuts to young children because of choking risk.

Frequently asked questions

What are the most common allergies?

The most common are pollen (hay fever), dust mites, pet dander and mold. Common food allergies include peanuts, tree nuts, milk, egg, shellfish, fish, wheat, soy and sesame. Insect stings, latex and medicines such as penicillin are other frequent triggers.

Can you develop allergies as an adult?

Yes. Allergies can appear at any age, including hay fever after moving to a new area, shellfish allergy in adulthood, and drug allergies. Some childhood allergies fade, while others start later. New symptoms deserve an allergist's assessment.

What is the difference between an allergy and an intolerance?

An allergy involves the immune system and can cause hives, swelling and, at worst, anaphylaxis. An intolerance, such as lactose intolerance, involves digestion and causes bloating, gas or diarrhea but is not dangerous. Testing can tell the two apart.

How is anaphylaxis treated?

Inject epinephrine into the outer thigh immediately, call 911, and lie the person down with legs raised unless breathing is easier sitting up. Give a second dose after 5 to 15 minutes if symptoms persist. Always go to the hospital, because symptoms can return.

Can allergies be cured?

Most allergies cannot be cured outright, but immunotherapy can reduce or remove sensitivity to pollen, dust mites, pets and insect venom over several years. Many children outgrow milk and egg allergies. Medicines control symptoms effectively for most people.