Inclusion criteria --Study participant must be minimum 18 years at the time of signing the informed consent form (ICF) --Study participant is lactating and will be at least 6 weeks postpartum on Day 1 of the study --Study participant has voluntarily decided, prior to having knowledge of this study, to cease breast milk feeding (by any means) in relation to her current period of lactation. Study participant agrees to cease or suspend breast milk feeding by Day 1 of the study and to not resume breast milk feeding (by any means) or donate expressed breast milk for 8 weeks following administration of rozanolixizumab. Participants may decide to resume breast milk feeding 8 weeks after administration of rozanolixizumab, with the agreement that any breast milk collected (to maintain milk supply) during that 8-week period will be discarded --Study participant is female A female participant is eligible to participate if she is not pregnant and at least 1 of the following conditions applies: ◦ A woman of childbearing potential (WOCBP) who agrees to follow the contraceptive guidance during the Sampling Period and for at least 1 week after the last dose of study treatment OR ◦ Not a WOCBP (ie, premenopausal female with documented hysterectomy, documented bilateral salpingectomy, or documented bilateral oophorectomy) Exclusion criteria --Study participant has history of breast implants, breast augmentation, or breast reduction surgery --Study participant has received live vaccine(s) within 4 weeks prior to Screening or plans to receive such vaccines during the study --Study participant has received treatment with biologic agents (such as monoclonal antibodies including marketed drugs) within 3 months or 5 half-lives (whichever is longer) prior to dosing --Study participant has had exposure to more than 3 new chemical entities within 12 months prior to dosing --Study participant has had an active clinically significant infection within the last 6 weeks