The objective of this study is to assess the safety, tolerability, pharmacokinetics and immunogenicity of either single ascending intravenous (IV) doses of ABBV-243 or single ascending subcutaneous (SC) doses of ABBV-243 in healthy adult participants (Part 1), and a single intravenous (IV) dose in healthy Asian adult volunteers (Part 2 There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.