Type I Allergy – Causes, Symptoms and Treatment

What Is a Type I Allergy?

A Type I allergy, also known as an immediate-type hypersensitivity reaction or IgE-mediated allergy, is the most common form of allergic reaction in humans. It occurs within minutes of exposure to an allergen and is driven by specific antibodies of the class immunoglobulin E (IgE). Common examples include hay fever, allergic asthma, food allergies, and insect venom allergy.

Causes and Development

A Type I allergy develops in two phases:

1. Sensitization Phase

Upon first contact with an allergen -- a substance that is harmless in itself, such as pollen, pet dander, or peanuts -- the immune system reacts excessively. It produces specific IgE antibodies directed against the allergen. These IgE antibodies attach to the surface of mast cells (in tissues) and basophil granulocytes (in the blood). During this phase, the affected individual does not yet experience any symptoms.

2. Effector Phase

Upon re-exposure to the same allergen, it binds to the IgE antibodies already anchored on the mast cells. This activates the mast cells, which within seconds to minutes release large amounts of histamine and other inflammatory mediators. These substances cause the characteristic allergic symptoms.

Common triggers of Type I allergy include:

• Pollen (grasses, trees, herbs)
• Animal dander (e.g., cat or dog hair)
• House dust mites
• Mold spores
• Foods (e.g., peanuts, milk, eggs, wheat)
• Insect venoms (e.g., bee or wasp stings)
• Latex
• Certain medications (e.g., penicillin)

Symptoms

The symptoms of a Type I allergy can affect multiple organ systems and vary depending on the type and amount of allergen as well as the route of exposure:

• Skin: Hives (urticaria), redness, itching, wheals, angioedema (swelling of deeper tissue layers)
• Eyes and nose: Itchy, watery eyes (allergic conjunctivitis), sneezing, runny nose (allergic rhinitis)
• Airways: Bronchospasm, shortness of breath, coughing, wheezing (allergic asthma)
• Gastrointestinal tract: Nausea, vomiting, abdominal cramps, diarrhea (in food allergies)
• Anaphylaxis: In the most severe cases, a life-threatening systemic reaction known as anaphylaxis can occur, characterized by a drop in blood pressure, loss of consciousness, and circulatory failure.

Diagnosis

Diagnosing a Type I allergy involves a combination of medical history, clinical examination, and specific allergy tests:

• Medical history: Assessment of symptoms, their timeline, and possible triggers.
• Skin prick test: The standard method, in which small amounts of various allergen extracts are applied to the skin and lightly scratched in. A wheal-and-flare reaction indicates sensitization.
• Specific IgE blood test (RAST/ImmunoCAP): Measurement of allergen-specific IgE antibodies in the serum.
• Total IgE: Elevated levels may suggest an allergic disposition but are not allergen-specific.
• Provocation test: Controlled exposure to the suspected allergen under medical supervision, e.g., as a nasal, bronchial, or oral challenge test.

Treatment

Allergen Avoidance

The most important measure is avoiding the triggering allergen (allergen avoidance), where possible. This significantly reduces the frequency and severity of allergic reactions.

Pharmacological Treatment

Several classes of medications are available to relieve symptoms:

• Antihistamines (H1 blockers): Block the effects of histamine and relieve itching, sneezing, and runny nose.
• Corticosteroids: Exert anti-inflammatory effects and are used for more severe reactions, e.g., as nasal sprays or systemic administration.
• Mast cell stabilizers (e.g., cromoglicic acid): Prevent the release of inflammatory mediators.
• Bronchodilators (e.g., beta-2 agonists): Used in allergic asthma to widen the airways.
• Adrenaline (epinephrine): The treatment of choice in anaphylactic shock, available as an emergency auto-injector.
• Biologics (e.g., omalizumab): Monoclonal antibodies that neutralize IgE, used in severe allergic asthma or chronic urticaria.

Allergen Immunotherapy (Desensitization)

Specific immunotherapy (SIT), also known as desensitization or hyposensitization, is the only causal treatment for Type I allergy. The allergen is administered in gradually increasing doses, either by subcutaneous injection or sublingually (under the tongue). The goal is to reprogram the immune response so that the immune system develops long-term tolerance to the allergen.

Distinction from Other Allergy Types

Type I allergy is one of four allergy types classified by Coombs and Gell:

• Type I (immediate type): IgE-mediated, reaction within minutes (e.g., hay fever, anaphylaxis)
• Type II (cytotoxic): IgG/IgM-mediated, destruction of the body's own cells (e.g., hemolytic anemia)
• Type III (immune complex): Deposition of antigen-antibody complexes (e.g., serum sickness)
• Type IV (delayed type): T-cell-mediated, reaction after 24 to 72 hours (e.g., contact dermatitis)

References

1. European Academy of Allergy and Clinical Immunology (EAACI): Allergy Guidelines, eaaci.org
2. Janeway, C.A. et al.: Immunobiology -- The Immune System in Health and Disease. 9th Edition. Garland Science, New York 2017.
3. World Allergy Organization (WAO): White Book on Allergy 2013 -- Update 2023. worldallergy.org