What round body of lymph tissue filters lymph as part of the immune system?

The lymph node is a critical component of the immune system, functioning as a filtering station for lymph fluid. Its primary role is to trap and destroy pathogens, such as bacteria and viruses, that are carried through the lymphatic system. This round body of lymphatic tissue is strategically located throughout the body and is densely populated with lymphocytes, which are white blood cells that play a significant part in the immune response.

When lymph fluid passes through a lymph node, it is monitored for any foreign substances; if pathogens or antigens are detected, the lymph node activates immune responses to combat them. This role makes lymph nodes essential for maintaining the body's defense mechanisms and helps in the prevention of infections and diseases.

The other options listed do serve important functions in the immune system and lymphatic system but do not serve the primary role of filtering lymph. The lymphatic duct is responsible for transporting lymph back into the bloodstream, while the spleen filters blood rather than lymph and helps recycle iron from red blood cells. The thymus is involved in the maturation of T-lymphocytes, which are crucial for adaptive immunity, but it does not filter lymph directly.