A 42-year-old woman presents to the emergency department with a 2-hour history of wheezing, urticaria, and throat tightness after eating shrimp at a restaurant. She has a history of similar reactions to shellfish in the past. Her vital signs show a blood pressure of 90/60 mmHg, heart rate of 110 beats/minute, respiratory rate of 24/minute, and oxygen saturation of 92% on room air. Physical examination reveals diffuse wheezes and raised, erythematous skin lesions. She is treated with intramuscular epinephrine, antihistamines, and corticosteroids, with symptom improvement.
Which of the following is the primary immunological mechanism responsible for her symptoms?
B) Type I hypersensitivity
The patient’s history of rapid-onset wheezing, urticaria, and throat tightness after shellfish exposure, along with response to epinephrine, suggests anaphylaxis due to type I hypersensitivity. This immunoglobulin E-mediated reaction involves allergen binding to immunoglobulin E on mast cells and basophils, triggering release of histamine and other mediators, causing urticaria, bronchospasm, and hypotension. Approximately 32% of adults have at least one type 1 hypersensitivity condition (allergic rhinitis, eczema, food allergy, or asthma), and about 10–15% of adults have multiple allergic conditions (e.g., asthma + rhinitis), with higher rates in women and those with a family history of atopy.
Answer choice A: Direct cytotoxic T-cell-mediated damage is incorrect. Cytotoxic T-cell-mediated damage involves T cells targeting infected or foreign cells, as in viral infections or graft rejection, and does not cause rapid allergic symptoms or urticaria.
Answer choice C: Type II hypersensitivity is incorrect. Type II hypersensitivity involves antibodies against cell surface antigens, as in hemolytic anemia or transfusion reactions.
Answer choice D: Type III hypersensitivity is incorrect. Type III hypersensitivity involves immune complex deposition, as in systemic lupus erythematosus or serum sickness, and causes subacute or chronic symptoms, not acute allergic reactions.
Answer choice E: Type IV hypersensitivity is incorrect. Type IV hypersensitivity is T-cell-mediated and delayed, as in contact dermatitis, and does not cause immediate symptoms like anaphylaxis or urticaria.
Key Learning Point
Type I hypersensitivity, mediated by immunoglobulin E, causes acute allergic reactions like anaphylaxis, with symptoms including urticaria, wheezing, and hypotension, triggered by allergens like shellfish. Treatment includes epinephrine, antihistamines, and corticosteroids, with avoidance of triggers for prevention.
