A lymph node is a bean-shaped solid structure along the ducts, or lymphatics, of the lymphatic system. The lymphatics run into internal conduits of the nodes, the cortical and medullary sinuses, which, in turn, empty into a draining vessel through one-way valves.
The sinuses are lined with macrophages, which can remove threats by phagocytosis. In addition, the nodes are rich in lymphocytes, which generate antibodies for the humoral response to invading substances.
In the presence of significant infection, lymphadenitis, or lymph node swelling, appears. The pattern of lymphadenitis, showing the areas from which infected lymph drains, helps locate the source of infection.
Early in the metastasis of cancer, malignant cells move from the immediate area of the tumor, into local lymph nodes, and then distant lymph nodes; the farther the cancer cells propagate, the more serious the stage of metastasis. That first local node is called the sentinel node and often is biopsied along with the tumor proper.