Anaphylactoid

Anaphylactoid Reactions

Anaphylactoid reactions are acute, systemic hypersensitivity reactions that are clinically indistinguishable from anaphylaxis, but differ in their underlying mechanisms. Unlike anaphylaxis, which is an IgE-mediated immune response, anaphylactoid reactions do not involve IgE antibodies and are instead triggered by direct mast cell and basophil activation.

Pathophysiology

Anaphylactoid reactions occur when certain substances directly cause the degranulation of mast cells and basophils, leading to the release of mediators such as histamine, prostaglandins, and leukotrienes. These mediators cause the symptoms associated with anaphylaxis, such as vasodilation, increased vascular permeability, and smooth muscle contraction.

Common triggers of anaphylactoid reactions include:

• Radiocontrast media
• Nonsteroidal anti-inflammatory drugs (NSAIDs)
• Opioids
• Vancomycin
• Exercise

Clinical Presentation

The clinical presentation of anaphylactoid reactions is similar to that of anaphylaxis and may include:

• Urticaria (hives)
• Angioedema
• Bronchospasm
• Hypotension
• Gastrointestinal symptoms such as nausea, vomiting, and diarrhea

Diagnosis

Diagnosis of anaphylactoid reactions is primarily clinical, based on the sudden onset of symptoms following exposure to a known trigger. Laboratory tests, such as serum tryptase levels, may be used to support the diagnosis, as elevated tryptase levels indicate mast cell activation.

Management

The management of anaphylactoid reactions is similar to that of anaphylaxis and includes:

• Immediate discontinuation of the triggering agent
• Administration of epinephrine
• Supportive care with oxygen therapy, intravenous fluids, and antihistamines
• Monitoring for potential complications

Prevention

Preventive measures for anaphylactoid reactions include:

• Avoidance of known triggers
• Pre-medication with antihistamines or corticosteroids in high-risk situations, such as prior to administration of radiocontrast media

See Also

• Anaphylaxis
• Hypersensitivity
• Mast cell

References

• Simons, F. E. R. (2010). Anaphylaxis. The Journal of Allergy and Clinical Immunology, 125(2), S161-S181.
• Lieberman, P., Nicklas, R. A., Oppenheimer, J., Kemp, S. F., Lang, D. M., Bernstein, D. I., ... & Wallace, D. (2015). The diagnosis and management of anaphylaxis practice parameter: 2015 update. The Journal of Allergy and Clinical Immunology, 136(3), 575-581.