A 2-year-old boy is brought to the emergency department due to wheezing and difficulty breathing. The patient had been trick-or-treating with his parents and ate several packs of candy containing peanuts. After he receives an intramuscular epinephrine injection, his symptoms resolve. At a follow up appointment, an allergy specialist places droplets of various allergens on the patient's skin and punctures the epidermis at each site. After 15 minutes, the skin at the site with peanut extract is erythematous with a raised, itchy bump that improves by the time the family leaves the office. Four hours later, the parents notice a hard, red swelling at the puncture site. Which of the following is most likely involved in this secondary reaction?

Question

A 2-year-old boy is brought to the emergency department due to wheezing and difficulty breathing.  The patient had been trick-or-treating with his parents and ate several packs of candy containing peanuts.  After he receives an intramuscular epinephrine injection, his symptoms resolve.  At a follow up appointment, an allergy specialist places droplets of various allergens on the patient's skin and punctures the epidermis at each site.  After 15 minutes, the skin at the site with peanut extract is erythematous with a raised, itchy bump that improves by the time the family leaves the office.  Four hours later, the parents notice a hard, red swelling at the puncture site.  Which of the following is most likely involved in this secondary reaction?

Quizplus · Accepted Answer

11ec160b_37ae_3e21_9249_e9e252a20180_MD0001_00 Following skin prick testing, this patient developed an erythematous, edematous welt that was followed 4 hours later by an indurated skin lesion. These findings are consistent with the early and late phases of a type I hypersensitivity reaction. After first exposure to an allergen (eg, peanuts), antigen specific IgE is produced by B-cells and binds to the surface of mast cells. If repeat exposure occurs, the bound IgE can cross-link and stimulate the release of preformed histamine and leukotrienes that cause vasodilation and increased capillary permeability. The result is a rapid (eg, minutes after exposure) early-phase type I hypersensitivity response characterized by superficial dermal edema and erythema (eg, wheal and flare reaction) that can progress to a more systemic response (eg, anaphylaxis) (Choice D). IgE also initiates the late phase of a type I hypersensitivity reaction by stimulating type 2 helper T cells to release cytokines (eg, IL-5) that activate eosinophils. Cationic proteins (eg, major basic protein, eosinophil peroxidase) released from eosinophils cause tissue damage, which usually manifests as a palpable, indurated lesion 2-10 hours following the early-phase reaction. (Choice A) Binding of preformed IgG antibodies to cell surface antigens initiates cell lysis mediated by complement and/or natural killer cells (type II hypersensitivity reactions). These reactions can develop hours to days after exposure to an antigen (eg, medications, transfused red blood cells), resulting in destruction of erythrocytes (causing fatigue, pallor), platelets (petechial bleeding), or leukocytes (fever, sepsis). (Choice B) Immune complex-mediated complement activation causes tissue damage days to weeks after initial antigen exposure in type III hypersensitivity reactions. Immune complexes tend to deposit in the basement membrane of small blood vessels in the skin, kidneys, or joints, resulting in vasculitis. (Choice E) In delayed (type IV) hypersensitivity reactions, CD4⁺ T cells release cytokines (eg, interferon gamma) that promote T cell- and macrophage-mediated tissue damage. Although these reactions can also present as an indurated skin lesion (eg, tuberculosis skin testing), type IV hypersensitivity reactions develop over days (rather than hours) because of the time needed for cellular amplification. Educational objective: The late phase of dermatologic type I hypersensitivity reactions manifests as an indurated skin lesion hours after exposure to the allergen due to local tissue damage caused by major basic protein released from eosinophils. In contrast, type IV hypersensitivity reactions develop over days because of the time needed to produce a cell-mediated immune response.

A 2-Year-Old Boy Is Brought to the Emergency Department Due