Small, rounded mass of lymphoid tissue contained in connective tissue .
They occur all along lymphatic vessels, with clusters in certain areas (e.g., neck, groin, armpits). They filter bacteria and other foreign materials out of lymph and expose them to lymphocyte s and macrophages that can engulf them; these cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infection s. The nodes also produce lymphocytes and lymphatic system . In Hodgkin disease and other lymphoma s, malignant lymph cells proliferate, causing lymph node enlargement. Other cancers often invade lymphatic vessels, which can carry cells from the tumour to lymph nodes, where they are trapped and grow into secondary tumours. Lymph nodes are therefore removed in cancer surgery to detect or prevent tumour spread.