Study Overview: This study is testing a new treatment for follicular lymphoma (FL), a type of blood cancer. The study compares two combinations of medicines: Mosunetuzumab and Lenalidomide (M + Len) versus Rituximab and Lenalidomide (R + Len). Participants must have previously tried at least one other treatment for their lymphoma.

Inclusion Criteria: Participants must have a specific type of FL and be in good enough health to participate. They need to have had previous lymphoma treatment and be willing to follow guidelines to prevent pregnancy during and after the study.

Exclusion Criteria: People with certain medical conditions, like severe allergies or other types of cancer, cannot participate. Those who have had recent treatments or certain infections are also excluded.

• Study Duration: This study involves multiple visits and treatments over time.
• Risks: There may be side effects from the medications used in this study.
• Compensation: Participants may receive compensation for their time and travel.