Final answer:
The lymph nodes are an additional organ in the lymphatic system, crucial for filtering lymph and initiating immune responses.
Step-by-step explanation:
An additional organ in the lymphatic system beyond the thymus gland, spleen, and red bone marrow is the lymph nodes. The lymph nodes are small, bean-shaped structures that are distributed along the lymphatic vessels throughout the body. They play a crucial role in the immune system by filtering lymph and providing a site where immune responses are initiated against pathogens. These nodes are rich in B and T lymphocytes and contain macrophages and dendritic cells for effective antigen presentation, which is critical for the activation of lymphocytes for the adaptive immune response.