What is the treatment for a Type 1 hypersensitivity reaction?

Treatment of Type 1 Hypersensitivity Reactions

Intramuscular epinephrine is the only first-line treatment for Type 1 hypersensitivity reactions (anaphylaxis) and must be administered immediately at 0.01 mg/kg of 1:1000 concentration (maximum 0.5 mg in adults, 0.3 mg in children) into the anterolateral thigh—antihistamines and corticosteroids should never delay or replace epinephrine. 1, 2, 3

Immediate Management Algorithm

Step 1: Recognize Anaphylaxis

Type 1 hypersensitivity reactions present with:

• Cutaneous manifestations: urticaria, angioedema, flushing, pruritus (present in most cases) 1
• Respiratory symptoms: dyspnea, wheezing, bronchospasm, stridor, laryngeal edema 1, 3
• Cardiovascular collapse: hypotension, tachycardia, syncope 1, 3
• Gastrointestinal symptoms: nausea, vomiting, abdominal cramping, diarrhea 3

Critical distinction: Differentiate from vasovagal reactions, which present with bradycardia (not tachycardia), pallor, and absence of cutaneous symptoms. 1

Step 2: Administer Epinephrine Immediately

Dosing 1, 2, 3:

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 concentration)
• Children <30 kg: 0.01 mg/kg (maximum 0.3 mg)
• Route: Intramuscular injection into the vastus lateralis (anterolateral mid-thigh) for optimal absorption 1, 2
• Repeat dosing: Administer every 5-15 minutes if symptoms persist or worsen 1, 4, 3

There are no absolute contraindications to epinephrine in anaphylaxis, including patients with cardiovascular disease, elderly patients, or those on beta-blockers. 1, 2 Delay in epinephrine administration is directly associated with fatalities and increased risk of biphasic reactions. 1, 2, 4

Step 3: Supportive Measures

• Position patient: Supine or Trendelenburg if hypotensive; position of comfort if respiratory distress present 4
• IV fluid resuscitation: Aggressive fluid administration for hypotension (mandatory) 4
• Supplemental oxygen: For respiratory symptoms 4
• Call for emergency assistance: Prepare for transport to emergency department 4

Step 4: Secondary Treatments (After Epinephrine)

H1 antihistamines (e.g., diphenhydramine 25-50 mg):

• Address only cutaneous symptoms (urticaria, pruritus, flushing) 2
• Slow onset of action (30 minutes to start, 60-120 minutes to peak) 2
• Do not prevent cardiovascular collapse or respiratory distress 2, 4
• Should only be given after epinephrine 2, 4

H2 antihistamines (e.g., ranitidine):

• No high-quality evidence supporting use in anaphylaxis 2
• May be combined with H1 blockers, though rigorous studies are lacking 2, 4

Bronchodilators (albuterol): For bronchospasm resistant to epinephrine 4

Glucagon: Consider in patients on beta-blockers with refractory symptoms 4

Critical Pitfalls to Avoid

Most dangerous error: Delaying epinephrine while administering antihistamines or corticosteroids first—this contributes to fatalities. 2, 4 Antihistamines cannot address life-threatening cardiovascular or respiratory manifestations. 2, 4

Corticosteroids have no role in acute anaphylaxis management:

• Slow onset of action (4-6 hours minimum) due to genomic mechanism 2
• Do not prevent biphasic reactions 2
• Should not be given with expectation of acute benefit 2, 4

Sedation from first-generation antihistamines can decrease awareness of worsening anaphylaxis symptoms. 2

Observation and Monitoring

• All patients must be observed until complete symptom resolution, regardless of initial severity 4
• Extended observation (6+ hours) or hospital admission required for: 4
  • Severe anaphylaxis
  • Patients requiring more than one dose of epinephrine
  • Risk factors for biphasic reactions
• Minimum 1 hour observation for patients without severe features before discharge 4
• Biphasic reactions can occur up to 72 hours after initial reaction 4

Special Considerations

For patients with β-lactam allergy (relevant in treatment contexts):

• Differentiate true immediate Type 1 hypersensitivity from other side effects 1
• Patients with immediate hypersensitivity may require desensitization or alternative agents 1
• Patients with non-immediate reactions may tolerate specific β-lactams 1

Intravenous epinephrine: Reserved only for cardiac arrest or profound hypotension unresponsive to multiple IM doses and IV fluids, requiring continuous hemodynamic monitoring. 4

Post-Event Management

• Prescribe epinephrine auto-injector for future reactions 4
• Refer to allergist for skin testing to identify triggers and develop emergency action plan 4
• Patient education on anaphylaxis recognition, trigger avoidance, proper auto-injector use, and biphasic reaction risk 4
• Medical identification bracelet should be considered 1