Can a hypersensitivity reaction occur upon initial exposure to an allergen?

From the Guidelines

Yes, it is possible to have a hypersensitivity reaction the first time you try an allergen, although this is rare and typically occurs in specific circumstances. The first exposure to an allergen usually serves as the sensitization phase, during which the immune system recognizes the substance and produces specific antibodies (usually IgE) against it 1. However, there are exceptions where prior cross-sensitization to similar allergens might cause what appears to be a first-time reaction, or when someone has been unknowingly exposed to an allergen previously 1. Some patients with anaphylaxis have low or undetectable circulating allergen-specific IgE, and anaphylaxis involves additional cell types that may include neutrophils, monocytes, macrophages, and platelets and signaling through mediators that include complement components, cysteinyl leukotrienes (LTs), platelet activating factor, IL-6, IL-10, and TNF-receptor 1 1. Key factors that contribute to the risk of anaphylaxis include the type of allergen, the dose, and individual susceptibility, with foods, especially peanut, tree nuts, milk, eggs, crustacean shellfish, and finned fish, being common triggers of anaphylaxis in the pediatric population 1. In cases where anaphylaxis is suspected, prompt administration of epinephrine is crucial, even if it's the first apparent exposure to the allergen, as the consequences of not treating anaphylaxis can be severe and life-threatening 1. It's also important to note that anaphylaxis can be difficult to distinguish from other diagnostic entities, and erring on the side of caution by injecting epinephrine and then observing the patient closely is often the best course of action 1. Overall, while rare, a hypersensitivity reaction can occur the first time an allergen is encountered, and it's essential to be aware of the risks and to take prompt action if symptoms of anaphylaxis appear. Some of the key points to consider include:

• The first exposure to an allergen is typically the sensitization phase, but exceptions can occur
• Prior cross-sensitization to similar allergens or unknowing previous exposure can lead to a reaction on first apparent exposure
• Anaphylaxis can involve multiple cell types and signaling pathways beyond just IgE
• Prompt epinephrine administration is critical in suspected anaphylaxis cases, regardless of the exposure history
• Distinguishing anaphylaxis from other conditions can be challenging, and caution should be exercised in diagnosis and treatment.

From the Research

Hypersensitivity Reactions to Allergens

• A hypersensitivity reaction can occur the first time an individual is exposed to an allergen, as an acute allergic reaction is a rapid-onset, IgE-mediated hypersensitivity reaction 2.
• This type of reaction can be caused by various factors, including food, insect stings, and medications, and can range from mild to severe symptoms, such as urticaria, swelling, abdominal cramping, and respiratory collapse 2.
• Anaphylaxis is a potentially life-threatening allergic reaction that can progress quickly, and treatment should be given as soon as possible 3.

Risk of Developing Food Allergy

• There is evidence to suggest that early introduction of allergenic foods may decrease the risk of developing IgE-mediated food allergy 4.
• A study found that regular infant egg consumption was associated with less egg allergy, and the association was suggestive for infant peanut consumption with zero peanut allergy cases 4.
• However, it is essential to note that anaphylaxis is a serious systemic hypersensitivity reaction that is usually rapid in onset and may cause death, and intramuscular adrenaline is the most important treatment 5.

Diagnosis and Management of Adverse Reactions to Food

• Food allergy is an aberrant immunological response to food antigen, which can result in potentially life-threatening reactions, and it is often challenging to differentiate food allergy from other adverse reactions to food 6.
• The diagnosis and management of adverse food reactions require a comprehensive approach, including classification, evaluation, and management of adverse food reactions, as well as the use of various food allergy testing 6.