Hypersensisitivity Type I: LATE RESPONSE.

Immediate (minutes): antigen crosslinks preformed IgE on presensitized mast cells

immediate degranulation > release of histamine (a vasoactive amine) and tryptase (a marker of mast cell activation).

Late (hours): chemokines (attract inflammatory cells, eg, eosinophils) and other mediators (eg, leukotrienes) from mast cells > inflammation and tissue damage.

FA 2020 P 112

This (late bee sting rxn) is describing an Arthus reaction (localized Type III HSR) + IgG immune complexes accumulate at site --> mast cell degranulation --> Neutrophil recruitment + IgG immune complexes also stimulate macrophages to release inflammatory cytokines (IL-1, IL-6, TNF-a) and chemokines (IL-8) --> Neutrophil recruitment result in edematous indurated lesion

Timeline for arthus reaction bee sting: 1-8 hours, generally: 6-12 hours vs Late phase Type 1 HSR 2-4 hours

https://www.sciencedirect.com/topics/immunology-and-microbiology/arthus-reaction

This NBME question is outdated; Based on a similar question in UWRLD these findings of urticaria followed by induration in a few hours are consistent with the early and late phases of a type I hypersensitivity reaction.

First exposure to an allergen antigen ->specific IgE is produced by B-cells and binds to the surface of mast cells.

Early phase: repeat exposure bound IgE cross-link and stimulate release of preformed histamine and leukotrienes that cause vasodilation and increased capillary permeability. The results superficial dermal edema and erythema (wheal and flare reaction) that can progress to a more systemic response (anaphylaxis).

Late phase: IgE stimulates type 2 helper T cells to release cytokines (IL-5) that activate eosinophils. Cationic proteins (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.

This is different from the induration we see in delayed (type IV) hypersensitivity reactions where CD4 T cells release cytokines (interferon gamma) that promote T cell- and macrophage-mediated tissue damage because type IV hypersensitivity reactions develop over days (rather than hours) due to the time needed for cellular amplification.

(FA 2020 Pg 214)

Acute inflammation + Vasodilation of vasculature and increased endothelial permeability. IL1, IL6, and TNFa are all involved in acute inflammation.