Allergic Reactions, Anaphylaxis, and Immunologic...

SECTION 1

Historical Context & Motivation

The understanding of allergic reactions and anaphylaxis has evolved significantly over the past century, transforming from poorly understood "strange diseases" into well-characterized immunologic cascades with clear treatment protocols. The word anaphylaxis itself derives from the Greek ana- (against) and phylaxis (protection), reflecting early researchers' surprise that the immune system could paradoxically harm the very organism it was meant to protect. For paramedics, mastery of this topic is non-negotiable because anaphylaxis is a true time-critical emergency where field interventions — particularly intramuscular epinephrine — remain the definitive life-saving treatment regardless of transport time.

1901

Discovery of Anaphylaxis

Charles Richet and Paul Portier, while studying sea anemone toxins, observed that a second exposure to the toxin caused fatal shock in dogs rather than immunity — coining the term anaphylaxis. Richet received the Nobel Prize in 1913 for this work.

1910

First Use of Epinephrine for Anaphylaxis

Clinicians began administering adrenaline (epinephrine) subcutaneously to reverse anaphylactic shock, establishing the pharmacologic cornerstone of treatment still used today.

1963

Gell and Coombs Classification

Philip Gell and Robin Coombs published their landmark classification of hypersensitivity reactions into four types (I–IV), providing the immunologic framework that guides clinical thinking about allergic and immune-mediated emergencies.

1987

EpiPen Auto-Injector Introduced

The commercialization of the epinephrine auto-injector empowered patients and bystanders to deliver first-line anaphylaxis treatment in prehospital settings, dramatically changing the paradigm of out-of-hospital care.

2020

Updated World Allergy Organization Guidelines

International consensus guidelines reinforced intramuscular epinephrine as first-line therapy, defined updated diagnostic criteria, and formally recognized anaphylaxis as underdiagnosed and undertreated worldwide.

Despite over a century of research, anaphylaxis remains a leading cause of preventable prehospital death. The central question this lesson addresses is: How does the paramedic rapidly differentiate among types of allergic and immunologic emergencies, and what evidence-based interventions can be initiated in the field to prevent cardiovascular collapse and death?

SECTION 2

Core Principles & Immunologic Foundations

Allergic reactions represent a spectrum of immune-mediated responses in which the body's defense mechanisms respond inappropriately to a normally harmless substance called an allergen (also referred to as an antigen in this context). The immune system's response is mediated by immunoglobulin E (IgE) antibodies that were produced during a prior sensitization event. Upon re-exposure, these IgE antibodies trigger mast cells and basophils to degranulate, releasing a cascade of potent chemical mediators — most notably histamine, leukotrienes, and prostaglandins — that produce the clinical signs and symptoms we observe. Understanding these foundational concepts allows the paramedic to predict the trajectory of an allergic event and intervene decisively.

Sensitization Phase

Initial exposure to an allergen causes B lymphocytes to produce allergen-specific IgE antibodies. These IgE molecules bind to the surface of mast cells and basophils, effectively "priming" them. No clinical symptoms occur during sensitization.

Re-exposure & Degranulation

Upon subsequent exposure, the allergen cross-links IgE antibodies on mast cells, triggering immediate degranulation. Histamine, leukotrienes, and prostaglandins flood the surrounding tissue, causing vasodilation, increased vascular permeability, and smooth muscle contraction.

Local vs. Systemic Response

A localized reaction produces symptoms confined to the site of exposure (e.g., hives, rhinitis). A systemic response (anaphylaxis) involves two or more organ systems and can progress to cardiovascular collapse within minutes.

Biphasic Reaction

Up to 20% of anaphylaxis cases exhibit a biphasic response — a recurrence of symptoms 1–72 hours after apparent resolution. This underscores the importance of hospital transport even when field treatment appears successful.

Anaphylactoid (Non-IgE) Reactions

Some agents (radiocontrast dye, NSAIDs, vancomycin) cause direct mast cell degranulation without IgE involvement. These anaphylactoid reactions are clinically indistinguishable from true anaphylaxis and are treated identically in the field.

✦ KEY TAKEAWAY

Think of sensitization like loading a spring-loaded trap: the first exposure sets and arms the mechanism silently, but the second exposure trips the trigger, releasing all the stored energy at once. In the body, that "stored energy" is the histamine and other mediators packed inside mast cells. The paramedic's job is to recognize the trap has been sprung — regardless of whether the mechanism was IgE-mediated or not — and deploy epinephrine to counteract the cascade before irreversible organ damage occurs.

SECTION 3

The Anaphylactic Cascade — Visual Explanation

This diagram illustrates the complete anaphylactic cascade from allergen exposure through IgE cross-linking, mast cell degranulation, and multi-organ effects. The green treatment box at the bottom emphasizes that epinephrine acts on multiple receptor subtypes simultaneously, making it the single most effective pharmacologic intervention against all pathologic pathways of anaphylaxis.

As depicted in the diagram, the clinical presentation of anaphylaxis is driven by the simultaneous activation of multiple organ systems. The cutaneous manifestations (urticaria, flushing, angioedema) are present in approximately 80–90% of anaphylaxis cases and are often the first clinical sign. However, the paramedic must recognize that anaphylaxis can occur without skin findings — particularly in patients who progress rapidly to cardiovascular collapse. The respiratory system is typically the most life-threatening component, as laryngeal edema can occlude the airway within minutes. Simultaneously, massive vasodilation and third-spacing of intravascular fluid produce distributive shock that can be refractory to standard fluid resuscitation alone, necessitating epinephrine's potent vasoconstrictive (α₁) and cardiac stimulant (β₁) properties.

SECTION 4

Pathophysiologic Mechanisms & Pharmacologic Intervention

Mediators of Anaphylaxis

The pathophysiology of anaphylaxis centers on three principal categories of chemical mediators released during mast cell and basophil degranulation. Histamine is the most rapidly released mediator, acting on H₁ receptors to cause vasodilation, increased capillary permeability, and bronchial smooth muscle contraction, and on H₂ receptors to stimulate gastric acid secretion and further vasodilation. Leukotrienes (particularly LTC₄, LTD₄, and LTE₄, collectively known as the slow-reacting substance of anaphylaxis) produce prolonged bronchospasm that is 100–1,000 times more potent than histamine on a molar basis and are resistant to antihistamine therapy. Prostaglandins (especially PGD₂) contribute to bronchoconstriction, vasodilation, and platelet aggregation. The combined effect of these mediators on the cardiovascular system produces a distributive shock pattern characterized by decreased systemic vascular resistance and relative hypovolemia due to capillary leak.

Epinephrine: Mechanism of Action

Epinephrine is the only medication that simultaneously reverses every major pathologic mechanism of anaphylaxis. Its effects are mediated through adrenergic receptors: α₁-receptor stimulation produces arterial vasoconstriction that reverses hypotension and reduces mucosal edema; β₁-receptor stimulation increases heart rate and myocardial contractility, improving cardiac output; β₂-receptor stimulation relaxes bronchial smooth muscle (bronchodilation) and, critically, stabilizes mast cell membranes to reduce further mediator release by increasing intracellular cyclic AMP (cAMP). The standard adult dose for anaphylaxis is 0.3–0.5 mg of 1:1,000 (1 mg/mL) concentration given intramuscularly into the lateral thigh (vastus lateralis), which provides faster and more reliable absorption than subcutaneous administration.

EPINEPHRINE DOSING — ADULTS

Epinephrine 1:1,000 → 0.3–0.5 mg IM (lateral thigh) q5–15 min PRN

1:1,000 = 1 mg/mL concentration. Repeat every 5–15 minutes as needed based on symptom severity. For refractory anaphylaxis with cardiovascular collapse, consider IV push epinephrine at 1:10,000 (0.1 mg/mL) in 0.1 mg increments or IV infusion at 2–10 mcg/min, titrated to effect.

EPINEPHRINE DOSING — PEDIATRICS

Epinephrine 1:1,000 → 0.01 mg/kg IM (max 0.3 mg) q5–15 min PRN

Weight-based dosing is essential in pediatric patients. For a 20 kg child: 0.01 × 20 = 0.2 mg IM. Use a length-based resuscitation tape (Broselow) if weight is unknown. Never delay epinephrine administration to obtain an exact weight.

💉 Clinical Pearl — Route Matters

Intramuscular (IM) injection into the vastus lateralis (lateral thigh) is the preferred route because the rich blood supply of skeletal muscle provides peak plasma levels within 8–10 minutes, compared to 30+ minutes with subcutaneous injection. Intravenous epinephrine is reserved for profound cardiovascular collapse or cardiac arrest due to the high risk of fatal dysrhythmias with rapid IV bolus. Always use 1:10,000 concentration for IV administration and give in small incremental doses.

SECTION 5

Classification of Hypersensitivity Reactions & Severity Grading

The Gell and Coombs classification divides hypersensitivity reactions into four types based on their underlying immunologic mechanism. While anaphylaxis (Type I) is the primary focus for paramedic-level care, understanding the broader classification system helps contextualize why certain immune-mediated emergencies present differently and require different management strategies. The table below provides a clinically oriented summary.

Gell and Coombs Classification of Hypersensitivity Reactions

Type	Name	Mechanism	Onset	Clinical Examples
Type I	Immediate / Anaphylactic	IgE-mediated mast cell degranulation	Seconds to minutes	Anaphylaxis, allergic asthma, hay fever, urticaria
Type II	Cytotoxic	IgG/IgM antibodies target cell surface antigens → complement activation	Hours to days	ABO transfusion reactions, hemolytic disease of newborn, Goodpasture syndrome
Type III	Immune Complex	Antigen-antibody complexes deposit in tissues → complement activation → inflammation	Hours to days	Serum sickness, lupus nephritis, rheumatoid arthritis
Type IV	Delayed / Cell-Mediated	T-cell mediated (no antibody involvement)	24–72 hours	Contact dermatitis (poison ivy), TB skin test reaction, transplant rejection

This three-column comparison illustrates the clinical spectrum from mild allergic reaction through anaphylaxis to anaphylactic shock. Notice how treatment escalates in parallel: antihistamines alone may suffice for mild reactions, but epinephrine is mandatory once anaphylaxis is suspected, and aggressive IV resuscitation with possible IV epinephrine is added for shock.

The clinical diagnostic criteria for anaphylaxis require one of three scenarios: (1) acute onset involving skin or mucosal tissue plus either respiratory compromise or hypotension; (2) two or more organ system involvement after exposure to a likely allergen (skin, respiratory, GI, cardiovascular); or (3) hypotension alone after exposure to a known allergen for that patient. In the prehospital setting, the practical approach is straightforward: if there is any doubt, treat as anaphylaxis. The risk of withholding epinephrine far exceeds the risk of administering it to a patient with a mild allergic reaction.

SECTION 6

Worked Example — Prehospital Anaphylaxis Management

Consider the following scenario: You respond as the lead paramedic to a 34-year-old female at a restaurant who reports feeling "tingly" and "tight in her throat" approximately 10 minutes after eating a dish containing shrimp. She has a known shellfish allergy but was unaware of the ingredient. She is anxious, has diffuse hives over her torso and arms, audible stridor, and states she feels like she "can't breathe." Vitals: HR 124, BP 88/52, RR 28, SpO₂ 91% on room air.

Systematic Anaphylaxis Management

Step 1 — Scene Safety & Primary AssessmentEnsure scene safety and don appropriate PPE. Form a general impression: the patient is in acute distress with respiratory compromise and signs of shock. Assess ABCs: the airway is compromised (stridor indicates laryngeal edema), breathing is labored with audible wheezing, and circulation reveals tachycardia and hypotension. This presentation meets criteria for anaphylaxis with impending cardiovascular collapse — two or more organ systems (skin, respiratory, cardiovascular) are involved after exposure to a known allergen.

Working Diagnosis: Anaphylaxis — Priority 1 / Immediate

Step 2 — Remove Allergen & Position PatientRemove any remaining food from the patient's mouth if present. Position the patient supine with legs elevated to maximize venous return and support blood pressure. If the patient is unable to tolerate supine positioning due to respiratory distress, allow a position of comfort (semi-Fowler's) but be aware this may worsen hypotension. Apply high-flow oxygen via non-rebreather mask at 15 L/min.

Patient supine, legs elevated; NRB at 15 L/min O₂

Step 3 — Administer Epinephrine IM (First-Line)Draw up 0.3–0.5 mg of epinephrine 1:1,000 (1 mg/mL) and administer via intramuscular injection into the lateral thigh (vastus lateralis). For this 34-year-old adult, administer 0.5 mg (0.5 mL) IM. Document the time of administration. This should be performed immediately upon recognition of anaphylaxis — do not delay for IV access or other interventions. Epinephrine addresses all major pathologic mechanisms: vasoconstriction (α₁), increased cardiac output (β₁), bronchodilation (β₂), and mast cell stabilization (β₂).

Epinephrine 0.5 mg IM administered at T+0 minutes

Step 4 — Establish IV Access & Administer AdjunctsWhile a partner monitors the patient, establish large-bore (16–18 gauge) IV access in one or both antecubital fossae. Initiate a normal saline bolus of 20 mL/kg (for a 70 kg patient, this is 1,400 mL) to counteract distributive hypovolemia. Administer adjunct medications per protocol: diphenhydramine 25–50 mg IV/IM (H₁ blocker), famotidine 20 mg IV (H₂ blocker if available), and initiate nebulized albuterol 2.5 mg for persistent bronchospasm. If methylprednisolone or dexamethasone is available per protocol, administer to help prevent the biphasic reaction, though corticosteroids have no acute benefit.

Two large-bore IVs established; 1,400 mL NS bolus initiated; diphenhydramine 50 mg IV given; albuterol neb running

Step 5 — Reassess, Repeat Epinephrine PRN, & TransportReassess vitals at 5-minute intervals. If symptoms persist or worsen after 5 minutes, repeat epinephrine 0.5 mg IM. Most patients require 1–3 doses. If the patient develops cardiac arrest or remains profoundly hypotensive despite IM epinephrine and fluids, consider transitioning to IV epinephrine (1:10,000 at 0.1 mg slow push or infusion at 2–10 mcg/min). Prepare for advanced airway management if stridor worsens — consider early intubation before complete airway obstruction occurs. Initiate rapid transport to the closest appropriate emergency department, providing a thorough radio report including allergen exposure, number of epinephrine doses, and current hemodynamic status.

Post-epi reassessment: HR 108, BP 96/64, SpO₂ 95% — improving; rapid transport initiated

SECTION 7

Common Allergens, Risk Factors, and Clinical Pitfalls

Understanding the most common allergens and recognizing high-risk patient populations allows the paramedic to maintain an elevated index of suspicion and anticipate deterioration. Equally important is awareness of the common clinical pitfalls that lead to delayed or inadequate treatment — errors that have been consistently documented in retrospective reviews of anaphylaxis-related deaths.

Common Allergens and Clinical Considerations for Prehospital Providers

Allergen Category	Common Examples	Route of Exposure	Clinical Notes
Foods	Peanuts, tree nuts, shellfish, milk, eggs, soy, wheat	Ingestion	Most common cause of anaphylaxis in children; onset 5–30 min after ingestion
Insect Stings	Hymenoptera: bees, wasps, hornets, fire ants	Envenomation	Most common cause of fatal anaphylaxis in adults; cardiovascular collapse often predominates over skin/respiratory symptoms
Medications	Penicillins, cephalosporins, NSAIDs, sulfonamides	Oral, IV, IM	IV route produces fastest/most severe onset; NSAIDs cause non-IgE (anaphylactoid) reactions
Latex	Natural rubber latex gloves, catheters, surgical devices	Contact, mucous membrane	Higher risk in healthcare workers, patients with spina bifida, those with multiple surgeries
Radiocontrast	Iodinated contrast media	IV	Anaphylactoid (non-IgE); typically occurs during first exposure; no prior sensitization required

Common Clinical Pitfalls in Anaphylaxis Management

Delayed epinephrine administration: This is the single most common and lethal error. Studies consistently show that delayed epinephrine is the primary risk factor for fatal anaphylaxis. If anaphylaxis is suspected, give epinephrine first and investigate second.
Substituting antihistamines for epinephrine: Diphenhydramine does not reverse bronchospasm, does not restore blood pressure, and does not stabilize mast cells. It relieves itching and hives only — it is an adjunct, never a replacement.
Failure to recognize anaphylaxis without skin findings: Approximately 10–20% of anaphylaxis cases present without urticaria or angioedema, particularly when cardiovascular collapse predominates.
Incorrect epinephrine concentration for the route: Using 1:1,000 (1 mg/mL) intravenously can cause fatal dysrhythmias. IM administration uses 1:1,000; IV administration requires 1:10,000 (0.1 mg/mL) in carefully titrated doses.
Premature discontinuation of monitoring: The biphasic response can produce a second wave of symptoms hours after apparent resolution. All anaphylaxis patients require transport and extended hospital observation.

✦ KEY TAKEAWAY

Think of anaphylaxis management like fighting a multi-alarm fire: you wouldn't send fire trucks to address only the smoke (antihistamines for hives) while ignoring the structural flames (bronchospasm, hypotension). Epinephrine is the fire engine that addresses the structural fire — everything else is supportive. The number one lesson from anaphylaxis fatality reviews is simple: epinephrine was given too late, or not at all.

SECTION 8

Special Populations & Advanced Considerations

Several patient populations present unique challenges in the assessment and management of anaphylaxis that the paramedic must anticipate. These include patients on beta-blockers, pregnant patients, pediatric patients, and elderly patients with comorbidities. Additionally, understanding the distinction between anaphylaxis and conditions that mimic it — such as vasovagal syncope, panic attacks, and acute asthma exacerbations — prevents both under-treatment and over-treatment in the field.

Special Populations and Differential Diagnosis Considerations

Population / Condition	Key Challenge	Modified Approach
Patients on Beta-Blockers	Beta-blockers blunt the compensatory tachycardia and may render epinephrine less effective by blocking β-receptors	Still give epinephrine IM; may require higher/more frequent doses. Consider glucagon 1–5 mg IV for refractory hypotension (bypasses β-receptors via cAMP pathway)
Pregnant Patients	Anaphylaxis threatens both mother and fetus; supine hypotensive syndrome may worsen shock	Epinephrine is NOT contraindicated in pregnancy — give standard dose. Position in left lateral decubitus to relieve aortocaval compression. Aggressive fluid resuscitation
Pediatric Patients	Weight-based dosing required; children may decompensate rapidly; may not verbalize symptoms	Epinephrine 0.01 mg/kg IM (max 0.3 mg); use Broselow tape if weight unknown; behavioral changes (irritability, lethargy) may be the earliest sign of shock
Elderly Patients	Higher risk of cardiac complications from epinephrine; polypharmacy (ACE inhibitors, beta-blockers) alters presentation	Still give epinephrine — benefits outweigh risks in true anaphylaxis. Use 0.3 mg IM initially; monitor cardiac rhythm closely; have IV access ready for arrhythmia management
Differential: Vasovagal Syncope	May mimic anaphylaxis with hypotension and loss of consciousness after injection or sting	Key differentiator: vasovagal causes bradycardia (not tachycardia), no urticaria, no bronchospasm, rapid resolution with supine positioning. If uncertain, treat as anaphylaxis

📋 NREMT Exam Focus

The NREMT frequently tests the concept that epinephrine is NEVER contraindicated in anaphylaxis, regardless of patient age, pregnancy status, or cardiac history. The risk of withholding it in true anaphylaxis (death from airway obstruction or cardiovascular collapse) always exceeds the risk of giving it (transient tachycardia, hypertension, or dysrhythmia). This is a foundational principle tested across multiple question formats.

Looking forward, advanced paramedic practice may increasingly incorporate point-of-care tools and expanded pharmacologic options for refractory anaphylaxis. Glucagon administration for beta-blocker-complicated anaphylaxis, vasopressin for catecholamine-refractory shock, and methylene blue as a nitric oxide scavenger for vasoplegic shock represent emerging considerations at the critical care transport level. While these are beyond the standard paramedic scope of practice, awareness of them reflects the evolving understanding of anaphylaxis pathophysiology and the limitations of current field treatment.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Explain why a patient experiencing anaphylaxis for the first time to a bee sting must have been exposed to Hymenoptera venom on a prior occasion, even if they have no recollection of a previous allergic reaction. What immunologic process makes this prior exposure necessary?

PROBLEM 2 — BASIC CALCULATION

A 25 kg pediatric patient is experiencing anaphylaxis after ingesting peanuts. Calculate the correct IM epinephrine dose using the standard weight-based formula of 0.01 mg/kg, and state the maximum single dose for a pediatric patient.

PROBLEM 3 — INTERMEDIATE

You administer 0.5 mg of epinephrine IM to a 45-year-old male with anaphylaxis after a wasp sting. After 5 minutes, his blood pressure improves from 78/40 to 94/58 mmHg, and his wheezing has partially resolved, but he still has significant stridor and an SpO₂ of 89%. Describe your next steps in order of priority.

PROBLEM 4 — APPLIED

You respond to a 68-year-old male who collapsed at a park after being stung by a bee. His wife states he takes metoprolol (a beta-blocker) for atrial fibrillation. He is unresponsive, has diffuse urticaria, no palpable radial pulse, and a carotid pulse of approximately 50 bpm. BP is unobtainable. Describe your assessment and treatment plan, explaining how his beta-blocker complicates management.

PROBLEM 5 — CRITICAL THINKING

A 22-year-old female presents with sudden-onset diffuse urticaria, facial flushing, tachycardia (HR 115), and a blood pressure of 82/48 mmHg approximately 15 minutes after receiving an IV infusion of vancomycin in an urgent care clinic. She has no known drug allergies and has never received vancomycin before. The nurse asks you whether this can really be anaphylaxis since 'she's never been exposed to the drug.' Provide a pathophysiologically grounded response explaining the mechanism at play, how it differs from true anaphylaxis, and whether your field management should differ.

SUMMARY

Lesson Summary

Allergic reactions exist on a clinical spectrum ranging from mild localized responses (urticaria, rhinitis) to life-threatening anaphylaxis and anaphylactic shock. The underlying mechanism of Type I hypersensitivity involves prior sensitization with IgE production, followed by re-exposure that triggers mast cell degranulation and the release of histamine, leukotrienes, and prostaglandins. Anaphylactoid reactions bypass the IgE pathway entirely but produce identical clinical effects and require the same treatment. Anaphylaxis is defined by multi-system involvement (skin, respiratory, cardiovascular, GI) and can progress to distributive shock within minutes.

The cornerstone of prehospital management is immediate intramuscular epinephrine (0.3–0.5 mg of 1:1,000 in adults; 0.01 mg/kg in pediatrics, max 0.3 mg), which simultaneously addresses vasoconstriction (α₁), cardiac stimulation (β₁), bronchodilation (β₂), and mast cell stabilization (β₂). Adjunct therapies include IV fluid resuscitation, antihistamines (H₁ and H₂), albuterol for bronchospasm, and corticosteroids to mitigate the biphasic response. Special populations — patients on beta-blockers, pregnant patients, pediatric and elderly patients — require awareness of modified presentations but never warrant withholding epinephrine. The single most important principle: when in doubt, give epinephrine — the risk of undertreating anaphylaxis is always greater than the risk of overtreating a mild allergic reaction.