Immunology · Year 2 · from Immunology
Case 1: Anaphylaxis (Type I Hypersensitivity)
Patient Demographics
- Age: 24 years old
- Sex: Male
- Ethnicity: Hispanic
Chief Complaint
Difficulty breathing and rash after eating at a restaurant
History of Present Illness
A 24-year-old man is brought to the emergency department by ambulance after developing difficulty breathing, hives, and throat tightness approximately 15 minutes after eating pad thai at a restaurant. He initially felt itching in his mouth and throat, which rapidly progressed to a generalized itchy rash and sensation that his tongue was swelling. He became lightheaded and short of breath. His girlfriend administered his epinephrine auto-injector before calling 911.
Past Medical History
- Peanut allergy diagnosed in childhood (prior mild reactions with hives)
- Asthma (mild intermittent)
- Allergic rhinitis
Medications
- Albuterol inhaler PRN
- Epinephrine auto-injector (carries but states has not needed in years)
Physical Examination
- General: Anxious, diaphoretic man in moderate respiratory distress
- Vital Signs: T 37.2C, HR 125, RR 28, BP 85/50, SpO2 92% on room air
- HEENT: Lip and tongue edema; uvula edema; stridor present; periorbital swelling
- Skin: Diffuse urticarial rash over trunk and extremities; flushing
- Lungs: Diffuse wheezing bilaterally; poor air movement
- Cardiovascular: Tachycardic, regular rhythm; no murmurs
- Abdomen: Mild cramping on palpation
Emergency Department Course
| Time | Intervention | Response |
|---|---|---|
| 0 min | EMS: IM epinephrine 0.3mg (pre-arrival) | Partial improvement |
| 15 min | Second IM epinephrine 0.3mg | HR 105, BP 95/60 |
| 15 min | IV fluids 1L NS bolus, diphenhydramine 50mg IV, methylprednisolone 125mg IV | Continued improvement |
| 45 min | Nebulized albuterol | Wheezing resolved |
| 4 hours | Monitoring | Stable, mild urticaria persists |
Clinical Image
Image illustrating urticarial wheals characteristic of Type I hypersensitivity reaction. These raised, erythematous, pruritic plaques result from mast cell degranulation and histamine release. Image source: Course lecture materials.
Diagnosis
Anaphylaxis due to peanut allergy (Type I IgE-mediated hypersensitivity)
Discussion
Anaphylaxis is a severe, potentially life-threatening systemic Type I hypersensitivity reaction:
Pathophysiology:
- Prior sensitization leads to allergen-specific IgE production
- IgE binds high-affinity FcepsilonRI receptors on mast cells
- Re-exposure: Allergen cross-links surface-bound IgE
- Mast cell degranulation releases preformed mediators (histamine, tryptase)
- Newly synthesized mediators (leukotrienes, prostaglandins) amplify response
Effects of mast cell mediators:
| Mediator | Effects |
|---|---|
| Histamine | Vasodilation, increased permeability, bronchoconstriction, pruritus |
| Leukotrienes C4/D4/E4 | Prolonged bronchoconstriction, mucus secretion |
| Prostaglandin D2 | Vasodilation, bronchoconstriction |
| Tryptase | Tissue damage (useful diagnostic marker) |
Anaphylaxis diagnostic criteria (2 or more):
- Skin/mucosal involvement (urticaria, angioedema, flushing)
- Respiratory compromise (bronchospasm, stridor, hypoxia)
- Cardiovascular (hypotension, end-organ dysfunction)
- Gastrointestinal (cramping, vomiting)
- After exposure to known or likely allergen
Why epinephrine is first-line:
- Alpha-1 agonist: Reverses vasodilation, reduces angioedema
- Beta-1 agonist: Increases cardiac output
- Beta-2 agonist: Bronchodilation, stabilizes mast cells
Treatment
- Immediate: Epinephrine IM (anterolateral thigh), repeated every 5-15 min if needed
- Supportive: IV fluids for hypotension, supplemental oxygen
- Adjuncts: Antihistamines, corticosteroids (prevent biphasic reactions)
- Monitoring: 4-6 hour observation for late-phase reactions
- Discharge: Epinephrine auto-injector prescription, anaphylaxis action plan
- Follow-up: Allergist referral for skin testing, education, and consideration of oral immunotherapy