Primary immune thrombocytopenia (ITP) is when your body mistakenly attacks platelets, which are cells that help stop bleeding. This study tests if a medicine called mezagitamab, given under the skin, can help keep platelet levels stable in adults with ITP compared to a placebo (a fake treatment that looks real but has no medicine in it). Participants will take mezagitamab for up to 6 months and visit the clinic several times. If they finish the main study or don't respond by week 16, they might join a follow-up study to keep receiving mezagitamab.

• The study lasts up to 6 months with several clinic visits.
• Participants may join a continuation study if they don't respond to treatment.
• Eligibility includes having ITP for at least a year and trying other treatments before.