Allergic Reactions: How Allergies Work and How They Are Treated

How allergic reactions happen

An allergy develops in two stages.

1. Sensitization: on first exposure, the immune system of a susceptible person makes IgE antibodies specific to the allergen. These antibodies attach to mast cells in the skin, airways and gut, and to basophils in the blood. There are no symptoms at this stage.
2. Reaction: on later exposure, the allergen binds to the IgE on mast cells and triggers them to burst open (degranulate), releasing histamine, tryptase, leukotrienes and other chemicals. These cause blood vessels to leak and widen, smooth muscle in the airways to tighten, and nerves to itch.

A late-phase reaction can follow 4 to 12 hours later as inflammatory cells move in, causing prolonged congestion, wheeze or swelling.

Why some people develop allergies is a mix of genes and environment. A family history of allergy, eczema or asthma (atopy) raises risk. Early skin inflammation, reduced microbial exposure in early life and delayed introduction of allergenic foods are thought to contribute.

Types of hypersensitivity reactions

Doctors classify immune overreactions into four types (Gell and Coombs classification):

When people say "allergic reaction" they usually mean type I. Severe delayed drug reactions such as Stevens-Johnson syndrome and DRESS are type IV and behave differently.

Common allergic conditions

• Allergic rhinitis (hay fever): sneezing, runny nose and itchy eyes from pollen, dust mites, mold or pets.
• Food allergy: the nine major US food allergens are milk, egg, peanut, tree nuts, fish, shellfish, wheat, soy and sesame.
• Drug allergy: penicillin is the most commonly reported, but over 90% of people labeled penicillin-allergic can take it safely after testing.
• Venom allergy: bee, wasp, hornet and fire ant stings.
• Latex allergy: more common in healthcare workers and people with many surgeries.
• Allergic asthma and atopic dermatitis (eczema), often in the same person.
• Allergic contact dermatitis: nickel, fragrances, preservatives, hair dye.
• Anaphylaxis: the most severe systemic form.

Symptoms and immediate first aid are covered on our page about allergic reaction symptoms.

Risk factors for severe reactions

• previous anaphylaxis
• asthma, especially if poorly controlled
• peanut, tree nut, fish and shellfish allergies
• adolescence and young adulthood (more risk-taking, delayed epinephrine)
• mast cell disorders
• beta-blockers or ACE inhibitors, which can make reactions harder to treat
• exercise, alcohol, NSAIDs and infection as co-factors

How allergies are diagnosed

A detailed history is the most important step: what was eaten or touched, how much, how soon symptoms began, what they were, and how they were treated. Tests are then used to confirm a suspected allergen rather than as a blind screen, because many people have positive tests without true allergy.

• Skin-prick test: a drop of allergen is pricked into the skin; a raised wheal after 15 minutes suggests sensitization. Antihistamines must be stopped for several days beforehand.
• Specific IgE blood test: measures antibodies to individual allergens; useful when skin testing is not possible.
• Component testing: identifies which protein within a food (for example, a peanut storage protein) is responsible, which helps predict severity.
• Patch testing: for delayed contact allergy; patches stay on the back for 48 hours.
• Oral food or drug challenge: the gold standard, giving gradually increasing amounts under medical supervision.
• Serum tryptase: taken within a few hours of a severe reaction to support a diagnosis of anaphylaxis.

Treatment

Avoidance

Identifying and avoiding triggers is the foundation: reading food labels, informing doctors and pharmacists of drug allergies, using mite-proof bedding, and keeping windows closed on high-pollen days.

Medicines

• Non-drowsy antihistamines (cetirizine, loratadine, fexofenadine) for itching, hives, sneezing.
• Steroid nasal sprays for persistent hay fever; most effective when used daily.
• Antihistamine or mast cell stabilizer eye drops.
• Topical steroids for allergic skin rashes.
• Epinephrine auto-injectors for anyone at risk of anaphylaxis. Carry two at all times and check expiry dates.
• Biologic medicines such as omalizumab (anti-IgE) for severe allergic asthma, chronic hives and, in some cases, multiple food allergies.

Allergen immunotherapy

Immunotherapy retrains the immune system by giving regular, gradually increasing doses of the allergen:

• Allergy shots or under-the-tongue tablets for pollen, dust mite and pet allergies, usually for 3 to 5 years
• Venom immunotherapy, which greatly reduces the risk of anaphylaxis from future stings
• Oral immunotherapy for peanut allergy in children, which raises the threshold for reactions

Prevention in children

Current guidance supports introducing peanut and egg in age-appropriate forms (never whole nuts) around 4 to 6 months of age, especially in babies with severe eczema, after discussing with the pediatrician. Treating eczema early may also reduce food allergy risk.

Outlook

Many childhood milk and egg allergies are outgrown by school age. Peanut, tree nut, fish and shellfish allergies usually persist. Hay fever often improves with age and with immunotherapy. With an action plan and epinephrine, deaths from anaphylaxis are rare.

Frequently asked questions

What causes an allergic reaction?

An allergic reaction happens when the immune system makes IgE antibodies against a harmless substance. On re-exposure, those antibodies trigger mast cells to release histamine and other chemicals, causing itching, swelling, sneezing, wheezing or, in severe cases, anaphylaxis.

What are the four types of allergic reactions?

Type I is immediate and IgE-driven (hay fever, food allergy, anaphylaxis). Type II involves antibodies attacking cells. Type III involves immune complexes depositing in tissues (serum sickness). Type IV is delayed and T-cell driven, such as contact dermatitis and many drug rashes.

How do doctors test for allergies?

Doctors start with a detailed history, then use skin-prick tests or specific IgE blood tests to confirm suspected allergens. Patch tests diagnose delayed contact allergies. A supervised oral challenge is the most accurate test for food or drug allergy.

Can allergies be cured?

Some can be greatly reduced. Allergen immunotherapy can provide long-lasting improvement for pollen, dust mite and venom allergies, and oral immunotherapy raises the reaction threshold for peanut. Many children outgrow milk and egg allergies. Other allergies are managed with avoidance and medicines.

Is an allergy the same as an intolerance?

No. An allergy involves the immune system and can cause hives, swelling and anaphylaxis. An intolerance, such as lactose intolerance, is a digestive problem that causes bloating, gas or diarrhea and is not life-threatening.