Type 1 Hypersensitivity Reactions

Definition and Mechanism

Type 1 hypersensitivity represents an acute IgE-mediated reaction that occurs when allergens cross-link IgE antibodies bound to FcεRI receptors on mast cells and basophils, triggering their degranulation 1
True allergic reactions are immune-mediated responses that occur reproducibly upon exposure to specific allergens at doses normally tolerated by most people 2, 3
The onset of symptoms typically occurs within minutes to hours (1-6 hours) after exposure to the allergen 2, 4

Histamine: Released within 5 minutes of mast cell activation and remains elevated for 15-60 minutes; causes vasodilation, increased vascular permeability, smooth muscle contraction, and mucus secretion 5
Tryptase: Released from mast cells during degranulation; blood samples for measurement are optimally obtained 15 minutes to 3 hours after onset of reaction 5

Urticaria (hives), pruritus (itching), flushing, and angioedema (swelling of lips, tongue, uvula) are common cutaneous manifestations of Type 1 hypersensitivity reactions 2, 3
Upper airway symptoms include rhinorrhea, nasal congestion, sneezing, laryngeal edema, and stridor 4
Lower airway symptoms include bronchospasm, wheezing, dyspnea, and reduced peak expiratory flow 4, 6
Hypotension, tachycardia, dizziness, syncope, and in severe cases, cardiovascular collapse are potential cardiovascular manifestations 1, 4
Nausea, vomiting, abdominal cramping, and diarrhea are possible gastrointestinal manifestations 3, 1
Anaphylaxis is a severe, potentially life-threatening systemic reaction involving multiple organ systems 1, 4

Epinephrine (adrenaline) is the first-line treatment for anaphylaxis, administered intramuscularly in the mid-outer thigh at a dose of 0.01 mg/kg (maximum 0.5 mg in adults, 0.3 mg in children) 4
Antihistamines (H1-blockers) are effective for cutaneous symptoms like urticaria and pruritus, but not a substitute for epinephrine in anaphylaxis 3
Corticosteroids may help prevent biphasic or protracted reactions, but have delayed onset of action 5
Bronchodilators are used for bronchospasm and wheezing 6

Allergen identification and avoidance are crucial for preventing recurrent reactions 2, 3
Prescription of epinephrine auto-injectors is recommended for at-risk patients with a history of anaphylaxis 4
Patient education on recognition and management of allergic reactions is essential 4
Referral to an allergist is recommended for comprehensive evaluation and management 2, 4

Type 1 hypersensitivity reactions must be distinguished from non-allergic hypersensitivity reactions that can mimic allergic symptoms 7
Non-allergic reactions include direct mast cell degranulation (anaphylactoid reactions), complement activation related pseudo-allergy (CARPA), and bradykinin-mediated angioedema 7

Delayed administration of epinephrine in anaphylaxis can lead to increased morbidity and mortality 4
Over-reliance on antihistamines alone for treatment of anaphylaxis is dangerous; epinephrine is the cornerstone of management 4
Beta-adrenergic blockers and angiotensin-converting enzyme inhibitors can increase the risk of severe anaphylactic reactions and may reduce the effectiveness of epinephrine 5
Measurement of tryptase levels during an anaphylactic episode followed by baseline measurement can help confirm mast cell involvement 5

The majority of severe systemic reactions begin within 30 minutes after allergen exposure, according to the World Allergy Organization SCIT Systemic Reaction Grading System 8
In food-induced anaphylaxis, death can occur within 30 minutes to 2 hours of exposure 9

70% of systemic reactions occur within 30 minutes after allergen exposure, as indicated by a review of literature 10
Most fatal reactions associated with allergen immunotherapy occurred within the first 30 minutes 8

Delayed systemic reactions can occur after 30 minutes and are generally not as severe 11
Biphasic reactions, where symptoms resolve and then recur, can happen in 1-23% of cases, typically within 24 hours of the initial reaction 11, 9

Patients receiving allergen immunotherapy should remain under medical observation for at least 30 minutes, as recommended by the European Academy of Allergy and Clinical Immunology 10
For food-induced anaphylaxis, a reasonable observation period after treatment is 4-6 hours due to the possibility of biphasic reactions 9

Poorly controlled asthma is a risk factor for severe immunotherapy-induced reactions 8
Adolescents and young adults with known food allergies and previous history of anaphylaxis are at highest risk for life-threatening food-induced anaphylaxis 9