ARGX-113-2009 is an operationally seamless 2-part, phase 2/3, prospective, global, multicenter, randomized, double-blinded, placebo-controlled study to investigate the efficacy, safety, tolerability, immunogenicity, participant-reported outcome measures (including those assessing participant QoL), PK, and PD of efgartigimod PH20 SC administered via subcutaneous (SC) injection in adult participants with moderate to severe BP. This study intends to demonstrate that efgartigimod is an effective and safe treatment for BP, providing participants with control of disease activity (CDA) and eventually remission while reducing their cumulative exposure to OCS. study will consist of 2 parts: * Part A of the study is a phase 2 evaluation that intends to provide proof of concept for the therapeutic activity of efgartigimod PH20 SC in participants with BP. * Part B of the study is a phase 3 evaluation that intends to confirm the results obtained from part A in a separate, larger group of participants with BP. An interim analysis will be performed during part A (on data obtained through week 26 for all Part A participants) to assess the primary endpoint and several secondary endpoints, confirm the appropriate sample size for part B of the study, and determine whether the efficacy results observed through week 26 of part A warrant continued study of efgartigimod PH20 SC for the treatment of participants with BP (futility analysis).