What’s a Fishbane reaction?

A Fishbane reaction is a mild, transient infusion reaction that occurs in roughly 1–3% of IV iron infusions, typically within the first few minutes of administration. It is not an allergic reaction but rather a benign form of complement activation–related pseudoallergy (CARPA) — an innate immune phenomenon triggered by nanoparticles of IV iron.

Mechanism

The reaction is thought to result from transient activation of the complement system when iron–carbohydrate complexes first enter the bloodstream. This leads to the release of anaphylatoxins (C3a, C5a), which briefly cause pulmonary vasoconstriction and stimulation of afferent autonomic pathways. The result is a sudden but short-lived sensation of pressure or tightness in the chest, back, or flanks, sometimes accompanied by flushing, warmth, anxiety, or tachycardia.

Unlike IgE-mediated allergy, no antibodies, mast cell priming, or sensitization are involved — and patients are not “allergic” to IV iron.

Clinical features

• Timing: Within 1–5 minutes of starting infusion
• Symptoms: Flushing, warmth, back or chest pressure, anxiety, sometimes mild dyspnea or tachycardia
• Duration: Usually resolves within 5–10 minutes after pausing infusion
• Severity: Mild, self-limited; does not progress to anaphylaxis
• Vital signs: May show transient tachycardia or mild hypotension, but no vascular collapse, urticaria, or bronchospasm

Management

1. Stop the infusion immediately.
2. Reassure the patient and observe until symptoms resolve (usually within minutes).
3. Restart at half the rate once asymptomatic; most patients tolerate resumption without recurrence.
4. No premedication with steroids or antihistamines is necessary and may obscure the diagnosis.
5. Future infusions can proceed normally, though many clinicians begin the next infusion slowly out of caution.

Clinical significance

Fishbane reactions are not a contraindication to future IV iron use. They represent the mildest expression of CARPA, distinct from both severe CARPA reactions and true anaphylaxis. Recognizing this prevents unnecessary labeling of patients as “iron allergic” and avoids withholding appropriate therapy.