How are lymph nodes like early warning systems?

• A. They monitor lymph with macrophages and lymphocytes for signs of trouble before it reaches vital organs
• B. They filter blood directly in the liver
• C. They secrete antibodies into the bloodstream
• D. They store iron for later use

Answer: (B)

Lymph nodes act as surveillance stations by filtering lymph from tissues and inspecting it for foreign invaders before it reaches the bloodstream. Inside, macrophages and other antigen-presenting cells grab pathogens and pass their antigens to B and T lymphocytes, which become activated right there and start the adaptive response. This local check creates an early warning system, catching trouble early in the lymph before it can spread to vital organs. Filtering blood in the liver isn’t what lymph nodes do—they filter lymph, not blood. Antibody secretion is carried out by plasma cells, mainly in lymphoid tissue and bone marrow, but the primary role of lymph nodes is detection and initial activation, not constant antibody output. Iron storage is handled by other cells and organs, not by lymph nodes.