The primary objective of the study is to evaluate the effects of a proton pump inhibitor (PPI) [rabeprazole] on BIIB122 pharmacokinetics (PK) after a single dose in healthy participants. The secondary objective of the study is to evaluate the safety and tolerability of BIIB122, with and without a PPI (rabeprazole), after a single dose in healthy participants.

Key Inclusion Criteria:

1. Body mass index between 18 and 30 kilograms per square meter (kg/^m2), inclusive.

3. Negative polymerase chain reaction test result for severe acute respiratory syndrome coronavirus (SARS-CoV-2) at Check-in.

4. For participants ≥ 55 years of age, ongoing conditions must be considered by the Investigator to be stable and adequately controlled by allowed concomitant medications.

Key Exclusion Criteria:

1. Plans to undergo elective procedures or surgeries at any time after signing the Informed Consent Form (ICF) through the follow-up visit.
2. Any condition affecting study treatment absorption.
3. History of gastrointestinal surgery, gastroesophageal reflux disease, or other clinically significant and active gastrointestinal condition per the Investigator's discretion.
4. Clinically significant acute gastrointestinal symptoms within 30 days prior to study Check-in (Day -1).
5. Chronic, recurrent, or serious infection, as determined by the Investigator, within 90 days prior to Screening or between Screening and Day -1.
6. For participants < 55 years of age: Use of any prescription medication (excluding oral contraceptives and hormone replacement therapy), OTC oral medications including proton pump inhibitors (PPI), histamine H2-receptor antagonists (H2 blockers), and antacids (excluding acetaminophen and aspirin ≤ 100 mg daily), or nutraceuticals including vitamins/mineral supplements/herbal medicines within 28 days prior to Day -1 and throughout the study.
7. For participants ≥ 55 years of age: Up to 5 medications are allowed if they are taken at a stable dose level and regimen.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/