The goal of this open label study is to measure pharmacokinetics, pharmacodynamics, early efficacy, and safety of tanruprubart in adult and pediatric participants, in the United States, Canada, and Europe.

Key Inclusion Criteria:

• Diagnosis of GBS according to the National Institute of Neurological Disorders and Stroke Diagnostic Criteria for GBS.
• Onset of GBS-related weakness ≤10 days before start of infusion on Day 1
• GBS-disability score (DS) score of 3, 4, or 5 at screening and before start of infusion on Day 1.

Key Exclusion Criteria:

• Previous or intended treatment with either plasma exchange or IV immunoglobulin for GBS.
• Diagnosis of a variant of GBS, including Miller Fisher syndrome, Bickerstaff's encephalitis, and overlap syndromes.

Other protocol-defined criteria may apply.