Allergy

How allergies work

1. Sensitization: on first exposure, the immune system wrongly identifies an allergen as a threat and makes IgE antibodies that attach to mast cells in the skin, airways and gut.
2. Reaction: on later exposure, the allergen binds these antibodies, and mast cells release histamine, leukotrienes and other chemicals within minutes.
3. Late phase: hours later, other immune cells arrive and prolong inflammation, which is why nasal congestion and asthma can persist.

Allergies tend to run in families. A child with one allergic parent has a higher chance of allergies, and higher still if both parents are allergic. People with this tendency (called atopy) often develop several allergic conditions over time, sometimes called the "allergic march": eczema in infancy, then food allergy, then hay fever and asthma.

Types of allergies

Allergic contact dermatitis works through a different immune pathway (T cells, not IgE), which is why it appears later and is tested with patch tests rather than skin prick tests.

Symptoms

Mild to moderate

• sneezing, runny or stuffy nose, itchy nose and palate
• itchy, red, watery eyes
• hives and itchy skin
• swelling of the lips or eyelids
• coughing and mild wheezing
• stomach cramps, nausea or diarrhea after a food

Anaphylaxis

Anaphylaxis is a severe, rapid, whole-body reaction, usually within minutes to 2 hours of exposure. It involves two or more body systems, or a sudden drop in blood pressure:

• skin: widespread hives, flushing, swelling
• breathing: throat tightness, hoarse voice, wheeze, shortness of breath
• circulation: dizziness, fainting, a weak fast pulse
• gut: vomiting, cramps, diarrhea
• a sense of doom

Epinephrine is the only first-line treatment. Antihistamines do not stop anaphylaxis. A second wave of symptoms (biphasic reaction) can occur hours later, so everyone treated for anaphylaxis should be observed in an ER.

What causes allergies to develop

Genes are a major factor, but rising rates of allergy in recent decades point to environmental influences:

• less early-life exposure to microbes (the hygiene or "old friends" hypothesis)
• delayed introduction of allergenic foods in infancy
• skin barrier damage from eczema, which allows sensitization through the skin
• air pollution and tobacco smoke
• climate change lengthening pollen seasons

Allergies can start at any age, including adulthood, and some, such as milk and egg allergy in children, are often outgrown.

Allergy testing

• Skin prick test: tiny amounts of allergens are pricked into the skin of the forearm or back. A raised itchy bump within 15 to 20 minutes suggests sensitization. Antihistamines must be stopped for several days beforehand.
• Specific IgE blood test: measures antibodies to particular allergens. Useful when skin testing is not possible.
• Component testing: identifies which proteins in a food you react to, which can predict severity (for example, in peanut allergy).
• Patch testing: for contact dermatitis, with patches left on the back for 48 hours.
• Oral food or drug challenge: eating or taking gradually increasing amounts under medical supervision. This is the gold standard for confirming or ruling out a food or drug allergy.

A positive test only shows sensitization, not necessarily allergy. Tests must be interpreted alongside your history. Broad "food panel" testing without symptoms often leads to unnecessary food avoidance.

Treatment

Avoidance

• Pollen: keep windows closed in high season, shower after being outdoors, check pollen forecasts.
• Dust mites: allergen-proof pillow and mattress covers, hot washing of bedding, low humidity.
• Pets: keep animals out of the bedroom, use HEPA filters.
• Food: read labels carefully and ask about ingredients when eating out.

Medicines

• Non-drowsy antihistamines (cetirizine, loratadine, fexofenadine) for sneezing, itching and hives.
• Steroid nasal sprays (fluticasone, mometasone, budesonide): the most effective treatment for allergic rhinitis, used daily.
• Antihistamine eye drops for itchy eyes.
• Leukotriene modifiers for allergic rhinitis with asthma, with attention to possible mood side effects.
• Short-term decongestants, avoiding them with high blood pressure.
• Biologic medicines such as omalizumab for severe allergic asthma, chronic hives and some food allergies.

Epinephrine auto-injectors

People with a history of anaphylaxis or at high risk carry two epinephrine auto-injectors (or nasal epinephrine) at all times and have a written emergency action plan.

Immunotherapy

• Allergy shots or under-the-tongue tablets for pollen, dust mite and pet allergies gradually retrain the immune system. A course lasts 3 to 5 years and can give lasting benefit.
• Venom immunotherapy is highly effective for insect sting anaphylaxis.
• Oral immunotherapy for peanut and other foods raises the threshold for reactions in selected children.

Prevention

• Introduce peanut and egg to babies early, around 4 to 6 months, as advised by your pediatrician. Babies with severe eczema or egg allergy should be assessed first.
• Treat eczema promptly to protect the skin barrier.
• Avoid smoking around children.

Frequently asked questions

What is an allergy?

An allergy is when the immune system overreacts to a normally harmless substance, such as pollen, food or pet dander. It produces IgE antibodies that trigger the release of histamine and other chemicals, causing symptoms like sneezing, itching, hives, swelling or wheezing.

What are the most common types of allergies?

The most common are hay fever (pollen), dust mite, pet and mold allergies, followed by food allergies, drug allergies, insect sting allergies, latex allergy and allergic contact dermatitis. Many people have more than one.

Can you develop allergies as an adult?

Yes. Allergies can start at any age. Adult-onset allergies to pollen, shellfish, medicines and insect stings are common. Some childhood allergies, such as milk and egg allergy, are often outgrown, while peanut, tree nut and shellfish allergies usually persist.

How do I know if I am having an allergic reaction?

Typical signs include sneezing, itchy or watery eyes, hives, itching, and swelling soon after exposure to a trigger. Trouble breathing, throat tightness, dizziness or swelling of the tongue are signs of anaphylaxis and need epinephrine and emergency care.

Can allergies be cured?

Most allergies cannot be cured, but they can be controlled. Allergen immunotherapy can reduce or switch off some allergies long term, especially to pollen, dust mites and insect venom. Many children outgrow milk and egg allergies.