This study is a multi-center, open-label study of intravenous (IV) ANX005 in participants with ALS.

In this Phase 2a, multi-center, open label, proof-of-biology study, ANX005 will be administered to participants with ALS.

The study consists of approximately 22 weeks of treatment and approximately 14 weeks of follow-up. All participants will be contacted by phone 6 months after study completion.

Key Inclusion Criteria:

• Diagnosis of ALS according to the World Federation of Neurology revised EI Escorial criteria.
• Onset of weakness within 3 years prior to Day 1 visit.
• Slow Vital Capacity ≥ 50% of predicted normal adjusted for sex, age, and height (from the sitting position).
• ALS Functional Rating Scale-Revised (ALSFRS-R) ≥ 30 at the Screening visit (Week -2).
• If female, must be postmenopausal, surgically sterilized, or childbearing potential must agree to use highly effective methods of contraception from Screening until 3 months after the last infusion with study medication.
• Males with a woman partner of childbearing potential must agree to use highly effective methods of contraception from Screening until Week until 3 months after the last infusion with study medication.
• Documented history of vaccinations within 5 years prior to Screening visit against encapsulated bacterial pathogens or willing to undergo vaccinations.

Key Exclusion Criteria:

• Clinically significant intercurrent illness, medical condition, or medical history that would jeopardize the safety of the participant, limit participation, or compromise the interpretation of the data derived from the participant.
• Participants with body weight > 150 kilograms.
• Antinuclear antibodies (ANA) titer ≥ 1:160 (for either of the 2 ANA results a minimum of 2 weeks apart) during the Screening Period.