Anaphylaxis is a systemic allergic reaction that involves the airway, vascular, gastrointestinal, and cutaneous systems. Severe symptoms can cause cardiac arrest and can be life-threatening.

Angioedema triggered by an allergic reaction to an insect sting.

Immune 

Anaphylaxis more precisely refers to antibody subclass immunoglobulins E (IgE) and G (IgG) hypersensitivity reactions. In these conditions, the individual would have had prior exposure to the allergen, leading to sensitization. At subsequent exposures, the allergen stimulates the reactive anaphylactic reaction. Often, however, patients will not be aware of prior exposure to the allergen. 

Anaphylactoid reactions behave similarly; however, they are not mediated by IgE related antibody reactions. The inciting “allergen” can be parenteral medications or contrast materials. Since the clinical symptoms and treatment are similar, it is not clinically necessary to distinguish the two reactions. In both cases, chemical mediators such as histamines, prostaglandin D2, tryptase, and leukotriene C mediate the downstream effects. 

Clinical

This is a systemic and rapid-acting allergic condition that causes angioedema, laryngeal edema, and bronchospasm with associated dyspnea. Severe anaphylaxis occurs if there is associated hypotension or lost consciousness. 

As many individuals with anaphylaxis may be unaware of prior exposure, it may be difficult to predict that a reaction will occur. However, elderly individuals, who have longer cumulative exposure to potential allergens, are at increased risk for these reactions. Nationwide research indicates that there was an increase from 21-25.1 patients per one million hospitalizes with an anaphylaxis reaction. Fortunately, the rate of death has decreased with under 1000 associated deaths per year. 

Underlying Etiology of Anaphylaxis

There is a wide range of potential allergens from insect bites or stings, contrast materials, and foods (i.e., peanuts, eggs, shellfish). Any allergen that triggers IgE can lead to anaphylaxis even exercise. There may be no identifiable allergen, and such patients may need to be managed with long-term corticosteroids. Beta blocking medications worsen anaphylaxis as they blunt the physiologic response to the excess adrenergic stimulation. 

Medications

Medications such as antibiotics (i.e., beta-lactam antibiotics including penicillin), NSAIDs, aspirin, and IV contrast materials are common precipitators for anaphylaxis

Latex

This is a commonly feared reaction, but it is increasingly rare, especially due to healthcare precautions. 

Insect Bites/Stings

Many insects from the Hymenoptera order (bees, ants, wasps, hornets, yellow jackets) can be triggers for anaphylaxis. This can lead to fatality within 10-15 minutes due to the collapse of the cardiovascular system. 

Foods

Common triggers are peanuts, tree nuts, seafood as well as wheat products. There has been increasing concern with the severity of nut-related anaphylaxis.

Peanuts can trigger food-induced anaphylaxis.

Pathophysiology

Anaphylaxis is caused by an immune response mediated to IgE. Once the allergen binds to its allergen-specific IgE (after prior sensitization), these reactions mediate the release of sensitive immune cells: mast cell and basophils. Note that in the anaphylactoid reaction, IgE mediation is not needed, and mast cell and basophils are released without activation of IgE. 

These reactions lead to a host of chemical mediators, including leukotrienes, histamines, thromboxane, prostaglandins, and bradykinins. The effects are vasodilation, increasing capillary leak, and constriction of the airway. Patients will present with angioedema, hypotension, and bronchospasm. A reaction near the time of exposure indicates increasing severity.