Adrenaline, Anaphylaxis, and Experimental Evidence

Perhaps uniquely among acute medical emergencies, the management of anaphylaxis is guided by strong clinical consensus rather than robust experimental evidence [1-4]. Since the earliest descriptions of anaphylaxis derived from animal models rather than controlled studies of human physiology our understanding of severe allergic reactions has evolved largely through clinical observation and mechanistic inference [5]. Consequently, intramuscular adrenaline, the universally recommended first-line therapy for anaphylaxis, has assumed clinical primacy despite limited direct experimental validation in humans [1-3]. The familiar doses of adrenaline used in current clinical practice can be traced to its earliest use as a purified hormone rather than robust randomized clinical trials [6]. In this context, the randomized, placebo-controlled study by Weiss-Tessbach and colleagues in this issue of Allergy represents an important attempt to interrogate a foundational therapeutic assumption using a controlled human model [7]. The investigators used a reproducible intravenous histamine infusion to induce hypotension in healthy volunteers, followed by intramuscular administration of adrenaline (300–500 μg) or placebo in exploratory and confirmatory phases. Histamine infusion produced marked reductions in mean arterial pressure (MAP), approximating the hemodynamic compromise observed in severe anaphylaxis, thereby enabling assessment of adrenaline's effects to reverse histamine-induced hypotension under controlled conditions. Despite achieving plasma adrenaline concentrations currently considered “therapeutic” in recent trials of adrenaline devices, intramuscular adrenaline failed to produce a sustained hemodynamic response in most participants [8]. Only a minority of participants demonstrated transient MAP recovery after a single injection, and repeat dosing did not consistently restore perfusion targets. In the confirmatory crossover analysis, the primary endpoint, area under the curve for MAP change, was not significantly different between adrenaline and placebo. At first glance, these findings appear to challenge established assumptions about adrenaline as a rescue therapy in anaphylaxis. However, their primary contribution is mechanistic clarification rather than overturning current therapeutic principles. Adrenaline's effect as a vasopressor remains undisputed, but anaphylaxis is not a single-mediator cardiovascular disorder. Rather, it is a multisystem syndrome in which a diverse array of mediators including Platelet-activating factor, leukotrienes, Prostaglandin D2, and nitric oxide drive airway compromise, vascular permeability, mediator amplification, and endogenous compensatory responses [9]. The experimental model conducted by Weiss-Tessbach et al. intentionally isolates histamine-mediated vasodilation, permitting physiological precision while necessarily excluding other defining features of clinical reactions. Participants developed hypotension without urticaria, angioedema, or bronchospasm, underscoring that cardiovascular responses alone capture only one dimension of anaphylaxis biology. Understanding adrenaline's actions requires consideration of its multiple receptor-mediated effects. Activation of α₁-adrenergic receptors on vascular smooth muscle promotes vasoconstriction, counteracting distributive shock. β₁-adrenergic stimulation enhances cardiac chronotropy and inotropy, supporting cardiac output during circulatory compromise. β₂-adrenergic receptors expressed on vascular smooth muscle additionally mediate vasodilation through cyclic AMP signaling, particularly within skeletal muscle vascular beds. Consequently, adrenaline exerts competing vascular effects, with the net hemodynamic consequence influenced by receptor distribution and circulating concentration [9]. Equally importantly, β₂-adrenergic receptors located in bronchial smooth muscle mediate bronchodilation, while β₂ signaling on mast cells inhibits further mediator release through cyclic AMP–dependent pathways [9]. Experimental work has demonstrated that adrenergic signaling can disrupt the cytoskeletal processes required for mast-cell degranulation, suggesting that adrenaline may limit the progression of anaphylaxis rather than simply reversing established hypotension [10]. These diverse receptor-specific effects imply that adrenaline's clinical benefit is mechanistically distinct both across organ systems and phases of a reaction in evolution. Viewed in this context, Weiss-Tessbach et al.'s findings suggest that plasma adrenaline concentrations achieved through intramuscular administration may be insufficient, in isolation, to overcome severe histamine-driven vasodilation. Peak plasma adrenaline concentration derived from a 500 mcg intramuscular injection as a pharmacokinetic target has not been derived from outcome-driven randomized controlled trials of clinical symptomatology. The modest hemodynamic response observed in this trial challenges reliance on peak plasma concentration as a surrogate marker of adrenaline's therapeutic adequacy for histamine-induced hypotension. This observation is particularly relevant as alternative delivery platforms are increasingly evaluated using pharmacokinetic equivalence rather than demonstrated physiological sequelae or randomized controlled trial outcomes. This study therefore represents an important shift from pharmacokinetics toward pharmacodynamics and organ-specific physiology. If adrenaline's major benefits arise partly through airway effects, modulation of mediator release, or support of endogenous compensatory mechanisms, then experimental models focused exclusively on vascular responses may underestimate its overall therapeutic impact. Conversely, such models provide valuable insight into which physiological domains are, and are not, influenced by standard dosing strategies. Equally significant is the methodological advance represented by this work. Allergy research has historically lacked reproducible experimental human systems capable of bridging mechanistic immunology and clinical therapeutics. Early insect-sting challenge studies provided important physiological observations but were constrained by unpredictability and safety concerns [11]. The controlled histamine challenge described here represents proof-of-concept that defined components of anaphylaxis physiology can be studied experimentally in humans under reproducible conditions. Such approaches will enable future investigations addressing timing of adrenaline administration, dosing thresholds, or comparison of delivery modalities, questions unlikely to be resolved through observational data alone. Intramuscular adrenaline remains the cornerstone of anaphylaxis management, supported by extensive clinical experience and biological plausibility. Real-world reactions involve heterogeneous mediators, ongoing antigen exposure, and patient populations with varying physiological reserve, none of which are reproduced in healthy volunteer models. The absence of a sustained vasopressor response in histamine-induced hypotension therefore, represents a mechanistic observation rather than evidence of clinical inefficacy. This study highlights a broader epistemological issue in allergy practice: many core therapies predate experimental validation because appropriate human models did not exist. The emergence of controlled experimental systems now permits long-standing assumptions to be examined empirically, strengthening rather than destabilizing clinical consensus. Ultimately, the primary value of this study is not in redefining adrenaline's role, but in refining our understanding of its mechanisms of action—it provides a foundation of trial design to challenge allergy dogma in anaphylaxis. If fatal anaphylaxis occasionally occurs despite repeated adrenaline administration, deeper physiological insight is required. Controlled human models—necessarily reductionist yet mechanistically informative—provide a vital bridge between immunological theory and best clinical care. In that sense, this work represents less a challenge to established therapy than an invitation to understand it more precisely. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.