## Discriminating Feature: Timing and Immunological Mechanism These two cases represent **Type I (IgE-mediated) hypersensitivity** versus **Type IV (delayed-type) hypersensitivity**, respectively. ### Type I Hypersensitivity (Immediate) **Key Point:** Onset within minutes to 1–2 hours; mediated by IgE and mast cell/basophil degranulation. - Positive skin prick test (IgE-mediated) - Urticaria, angioedema, anaphylaxis - Requires preformed mediators (histamine, tryptase) ### Type IV Hypersensitivity (Delayed) **Key Point:** Onset 24–72 hours or later; mediated by T-lymphocytes (CD8+ and CD4+), not antibodies. - Negative skin prick test - Positive lymphocyte proliferation assay (T-cell activation) - Maculopapular rash, contact dermatitis - Requires antigen processing and T-cell recognition ### Comparison Table | Feature | Type I (IgE) | Type IV (T-cell) | | --- | --- | --- | | **Onset** | Minutes–2 hours | 24–72 hours or later | | **Mediator** | IgE, mast cells, histamine | T-lymphocytes, cytokines | | **Skin prick test** | Positive | Negative | | **Lymphocyte test** | Negative | Positive | | **Clinical signs** | Urticaria, angioedema, wheezing | Rash, eczema, contact dermatitis | **High-Yield:** The **timing of onset** (immediate vs. delayed) is the single best discriminator. Type I reactions occur within minutes because IgE is pre-bound to mast cells; Type IV reactions require T-cell priming and take 24+ hours. **Clinical Pearl:** Amoxicillin rashes on day 7 are classically Type IV hypersensitivity (maculopapular exanthem), whereas anaphylaxis within 15 minutes is Type I. [cite:Robbins 10e Ch 6] --- ## Why Option 0 is Correct Timing and the presence of IgE-mediated mast cell degranulation are the **hallmark discriminators**. Type I is immediate and IgE-driven; Type IV is delayed and T-cell–driven. This distinction is both mechanistic and clinically actionable.
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