Systemic Shock: Anaphylaxis and Allergy Quiz

Anaphylaxis is a severe, rapidly progressing systemic allergic reaction that can be life-threatening. It is driven by the simultaneous activation of sensitized mast cells and basophils throughout the body following systemic allergen exposure. Massive release of histamine, leukotrienes, and other mediators causes vasodilation, vascular leakage, bronchoconstriction, and cardiovascular collapse, constituting a medical emergency requiring immediate treatment.

Epinephrine is the cornerstone treatment for anaphylaxis. It works through multiple mechanisms simultaneously. By activating alpha-adrenergic receptors, it causes vasoconstriction and raises blood pressure. Beta-2 adrenergic receptor activation relaxes bronchial smooth muscle, reversing bronchoconstriction. It also reduces mast cell mediator release. Intramuscular injection into the thigh is the recommended route for rapid absorption in emergency situations.

The most common triggers of severe anaphylaxis include peanuts, tree nuts, shellfish, fish, cow milk, eggs, insect venom such as from bees and wasps, and medications including penicillin and NSAIDs. These allergens are capable of triggering widespread IgE-mediated mast cell and basophil activation across multiple organ systems simultaneously, producing the life-threatening systemic reaction characteristic of anaphylaxis.

Histamine, rapidly released from mast cell and basophil granules during anaphylaxis, acts on multiple tissues through H1 and H2 receptors. It causes vasodilation and increased vascular permeability, leading to fluid leakage, urticaria, and hypotension. In the airways, histamine induces bronchoconstriction and mucus secretion. These combined cardiovascular and respiratory effects contribute significantly to the life-threatening manifestations of anaphylactic shock.

Anaphylaxis is a systemic reaction affecting multiple organ systems simultaneously. The cardiovascular system is impacted through massive vasodilation and increased vascular permeability, causing hypotension and distributive shock. The respiratory system is affected through bronchoconstriction and laryngeal edema, which can be fatal if untreated. The skin typically manifests urticaria, flushing, and angioedema. Gastrointestinal symptoms such as nausea and cramping may also occur.

Biphasic anaphylaxis is a well-documented phenomenon in which a second anaphylactic reaction recurs 1 to 72 hours after the initial reaction has apparently resolved, without further allergen exposure. It is thought to result from the late-phase inflammatory response involving leukotrienes and newly recruited immune cells. This is why patients treated for anaphylaxis are recommended to be observed in a clinical setting for several hours after initial treatment.

Although IgE-mediated anaphylaxis and anaphylactoid reactions produce clinically identical symptoms, their underlying mechanisms differ. IgE-mediated anaphylaxis requires prior sensitization and allergen-specific IgE. Anaphylactoid reactions involve direct mast cell and basophil activation without IgE involvement, triggered by agents such as contrast media, opioids, and certain drugs. Both conditions require the same emergency management with epinephrine.

Leukotrienes, particularly LTC4, LTD4, and LTE4, are potent lipid mediators synthesized by mast cells, eosinophils, and basophils via the arachidonic acid pathway. In allergic asthma, they cause prolonged bronchoconstriction that is more sustained than histamine-driven constriction, increased mucus secretion, airway edema, and recruitment of eosinophils. Leukotriene receptor antagonists such as montelukast are used clinically to manage allergic asthma and allergic rhinitis.

Several factors increase the risk of severe or fatal anaphylaxis. Pre-existing asthma is a significant risk factor because airway hyperreactivity worsens bronchoconstriction during anaphylaxis. A history of prior anaphylactic reactions indicates established IgE sensitization and heightened mast cell reactivity. Elevated baseline serum tryptase, which may indicate mastocytosis, is associated with more severe reactions due to increased total mast cell burden.

Venom immunotherapy is a highly effective form of allergen desensitization for individuals with IgE-mediated allergy to bee or wasp venom. By administering gradually increasing doses of venom allergen, the immune response is shifted away from Th2 and IgE production toward regulatory T cell activity and IgG4 production. This significantly reduces the risk of anaphylaxis upon future stings, and is recommended for individuals with a history of systemic venom-induced anaphylaxis.

Angioedema is swelling caused by increased vascular permeability and fluid leakage into deeper layers of the skin, subcutaneous tissues, and mucous membranes. During anaphylaxis, histamine and other vasoactive mediators drive angioedema formation. When it involves the larynx or pharynx, it can cause life-threatening airway obstruction. Rapid recognition and epinephrine administration are critical to preventing fatal airway compromise in anaphylaxis.

After epinephrine, second-line agents used in anaphylaxis management include antihistamines such as diphenhydramine to reduce histamine-mediated symptoms, and corticosteroids such as methylprednisolone to suppress late-phase inflammation and reduce the risk of biphasic reactions. Intravenous fluids address distributive shock from vasodilation, and bronchodilators may be needed for persistent bronchospasm. These agents support but do not replace epinephrine as the primary intervention.

The atopic march describes the characteristic progression of allergic diseases over a lifetime. It typically begins with atopic dermatitis in infancy, followed by food allergies, then allergic rhinitis, and eventually asthma. This progression reflects expanding IgE sensitization to multiple allergens over time. A disrupted skin barrier, as seen in filaggrin gene mutations, is thought to allow allergen entry and sensitization that initiates this sequential development of atopic conditions.

Mast cell stabilizers such as cromolyn sodium work by preventing mast cell degranulation, not by blocking IgE production. They stabilize mast cell membranes and inhibit the release of preformed and newly synthesized inflammatory mediators in response to allergen-IgE cross-linking. They are used prophylactically to reduce allergic symptoms in conditions such as allergic rhinitis and asthma but do not address the underlying IgE-mediated sensitization process.

Serum tryptase is a protease stored in mast cell granules and released during degranulation. Measuring tryptase levels within 1 to 3 hours of an anaphylactic event provides objective laboratory evidence of mast cell activation. Elevated tryptase supports the clinical diagnosis of anaphylaxis, particularly in cases where the presentation is atypical. Baseline tryptase levels should also be measured after recovery to screen for underlying mastocytosis.