Type 1 hypersensitivity, also known as immediate hypersensitivity, is an allergic reaction that occurs rapidly after exposure to an allergen. It is mediated by IgE antibodies and results in the release of inflammatory mediators like histamine from mast cells and basophils. This leads to symptoms like hives, swelling, itching, sneezing, wheezing, and anaphylaxis. Some common examples of type 1 hypersensitivity include allergies to foods like peanuts, tree nuts, shellfish, eggs, and milk, as well as environmental allergens like animal dander, pollen, dust mites, and insect stings.
Characteristics of Type 1 Hypersensitivity
Type 1 hypersensitivity has the following key characteristics:
- It occurs rapidly, usually within minutes after exposure to the allergen.
- It is mediated by IgE antibodies that bind to Fc receptors on mast cells and basophils.
- Cross-linking of IgE by allergens triggers mast cell and basophil degranulation, releasing inflammatory mediators like histamine.
- Symptoms involve tissues near where the reaction occurs and can include hives, angioedema, rhinitis, asthma, gastrointestinal distress, and anaphylaxis.
- Repeated exposure causes more severe symptoms due to increased IgE levels and sensitized mast cells.
Mechanism of Type 1 Hypersensitivity
The mechanism of a type 1 hypersensitivity reaction can be summarized in the following steps:
- Initial exposure: The allergen first enters the body and is taken up by antigen presenting cells like dendritic cells. These cells process the allergen and present peptides to naive T cells.
- T cell activation: The allergen peptide stimulates allergen-specific CD4+ T helper 2 (Th2) cells, causing them to activate and proliferate. Activated Th2 cells secrete cytokines like IL-4, IL-5, IL-13.
- IgE production: B cells are stimulated by the Th2 cytokines to undergo class switching to produce allergen-specific IgE antibodies.
- IgE sensitization: The IgE antibodies bind to high affinity Fc receptors (FcεRI) on the surface of mast cells and basophils, sensitizing them.
- Allergen re-exposure: When the allergen is encountered again, it cross-links the IgE on sensitized mast cells and basophils, activating them.
- Cell activation: Activated mast cells and basophils rapidly degranulate, releasing inflammatory mediators like histamine, leukotrienes, and cytokines.
- Symptoms: These mediators act on local tissues causing vasodilation, mucus secretion, muscle contraction, and other effects that lead to type 1 hypersensitivity symptoms.
Examples of Type 1 Hypersensitivity
Some of the most common examples of type 1 hypersensitivity reactions include:
Food Allergies
- Peanut allergy – One of the most severe and common food allergies, especially in children. Symptoms include hives, throat tightness, difficulty breathing, and anaphylaxis.
- Tree nut allergy – Allergy to nuts like almonds, walnuts, cashews. Symptoms similar to peanut allergy.
- Milk allergy – Common in children who may outgrow it. Can cause hives, vomiting, diarrhea, wheezing.
- Egg allergy – Also common in kids. Symptoms include skin rash, vomiting, and anaphylaxis.
- Shellfish allergy – Allergy to crustaceans like shrimp, lobster, crab. Can cause severe reactions.
Environmental Allergies
- Seasonal allergic rhinitis – Hay fever symptoms caused by pollen allergies. Sneezing, congestion, runny nose, itchy eyes.
- Dust mite allergy – Year-round allergic rhinitis and asthma caused by dust mite feces that trigger IgE.
- Animal dander allergy – Allergy to proteins in skin flakes, saliva of pets with fur or feathers. Causes rhinitis and asthma.
- Insect sting allergy – Venom from stinging insects like bees, wasps, hornets, fire ants. Can induce anaphylaxis.
Drug Allergies
- Penicillin allergy – Hives, rash, swelling, and anaphylaxis. Most common drug hypersensitivity.
- NSAID allergy – Allergy to nonsteroidal anti-inflammatory drugs like ibuprofen. Causes hives, facial swelling.
Diagnosis of Type 1 Hypersensitivity
Type 1 hypersensitivity is diagnosed through:
- Allergy skin testing – Prick/scratch testing extracts of suspected allergens on the skin. A positive result is a raised itchy wheal.
- Serum allergen-specific IgE tests – Measuring levels of IgE to specific allergens in the blood. Higher levels indicate an allergy.
- Oral food challenge – Gradually feeding suspected food allergens to confirm an allergy. Done in a medical setting due to risk of reaction.
- Elimination diet – Removing suspected allergens from the diet to see if symptoms improve. Useful for conditions like eosinophilic esophagitis.
Other helpful tests include a complete blood count to check for elevated eosinophils and serum tryptase levels during an acute reaction.
Treatment of Type 1 Hypersensitivity
Treatment of type 1 hypersensitivity involves:
- Allergen avoidance – Avoiding exposure to known allergenic triggers as much as possible.
- Antihistamines – Blocking the histamine released during reactions using medicines like diphenhydramine, cetirizine, or fexofenadine.
- Corticosteroids – Using steroids like prednisone to reduce inflammation during acute reactions.
- Bronchodilators – Opening airways using medicines like albuterol for conditions like allergic asthma.
- Epinephrine – Administering epinephrine by autoinjector during anaphylaxis to reverse symptoms.
- Immunotherapy – Gradually administering increasing doses of allergens to induce tolerance. Used for environmental allergies.
For severe food allergies, avoiding the food trigger completely is essential. Carrying emergency epinephrine is vital for those at risk of anaphylaxis.
Prevention of Type 1 Hypersensitivity
Preventing type 1 hypersensitivity involves:
- Breastfeeding infants exclusively for 4-6 months to avoid food allergens.
- Introducing high-risk foods like eggs, fish, peanuts early after starting solids around 4-6 months.
- Avoiding tobacco smoke exposure in children.
- Monitoring for symptoms of food protein-induced enterocolitis syndrome.
- Pet exposure should be avoided in high-risk infants. Exposure after 1 year may decrease pet dander allergy risk.
- Getting venom immunotherapy after a severe insect sting reaction.
Research into food allergy prevention is ongoing, including evaluating early peanut introduction and possible use of peanut oral immunotherapy. More studies are needed to determine optimal strategies.
Conclusion
Type 1 hypersensitivity is an IgE-mediated allergic reaction that occurs rapidly after allergen exposure. It is characterized by release of inflammatory mediators from mast cells and basophils, causing symptoms in nearby tissues. Common examples include food allergies to eggs, milk, nuts, and shellfish, environmental allergies to animal dander and pollen, as well as drug and insect sting allergies. Diagnosis involves allergy testing and treatment focuses on allergen avoidance, antihistamines, epinephrine for anaphylaxis, and immunotherapy. Further research is needed to determine optimal prevention strategies to reduce the burden of type 1 hypersensitivity disorders like food allergies.
