In which structure of the lymphatic system is the lymph actually cleaned?
Lymph nodes, mostly in the armpits, neck and groin.
Lymph is a clear liquid derived from blood plasma. Once plasma has left a cell, having delivered its nutrients, it returns partly to the venous system and partly into the lymphatic system.
Lymph contains large amounts of WBC's, such as the eponymous lymphocytes.
The main role of the lymphatic system is to clear toxins and waste, which happens in lymph nodes: small, bean-shaped centers on the lymphatic vessels which clear the lymph as it is being carried from the extremities of the body back to join with the blood stream around the neck.
Lymph nodes also produce and store WBC's in order to fight off infection when and where needed.