Which term describes an immediate allergic reaction?

• A. Anaphylaxis
• B. Allergen
• C. Sepsis
• D. Type I Hypersensitivity

Answer: (D)

Immediate allergic reactions are driven by IgE antibodies that bind to allergens and sensitize mast cells and basophils. When the allergen encounters these sensitized cells again, it cross-links the IgE on their surfaces, triggering rapid degranulation and release of histamine and other mediators. This sudden surge causes the quick symptoms we associate with allergies, such as itching, redness, swelling, runny nose, and bronchoconstriction, typically within minutes of exposure.

This mechanism defines Type I hypersensitivity. While anaphylaxis is a severe, systemic manifestation of the same IgE-mediated reaction, the general term for an immediate allergic reaction is Type I hypersensitivity. The other terms describe different concepts: an allergen is the trigger, sepsis is an infection, and anaphylaxis is a potentially life-threatening manifestation of the immediate allergic response rather than the broad category itself.