Follicular Lymphoma (FL) is a type of cancer that affects the lymphocytes, which are cells in the immune system. This study is testing a new treatment involving three drugs: epcoritamab (an experimental drug), rituximab (an intravenous drug), and lenalidomide (an oral drug). The study will check if these drugs together are safe and effective for adults whose FL has returned or hasn't responded to treatment. About 500 adults from around the world will take part.

Participants will receive different treatments depending on their group. Some will receive R2 alone, which includes rituximab and lenalidomide, while others will receive these drugs along with epcoritamab for up to 12 cycles, where each cycle lasts 28 days.

Participants will visit the clinic regularly for medical tests, blood tests, and to answer questions about side effects.

• Participants may face a higher treatment burden than usual care.
• Regular hospital/clinic visits are needed for check-ups.
• The study involves up to 12 cycles of treatment.