Located at intervals along the lymphatic vessels that conduct lymph back to the venous circulation are lymph nodes.  The lymph nodes act as filters to remove microorganisms and other foreign substances from the lymph that is passing through them.  Within lymph nodes are phagocytic macrophages that engulf and destroy microorganisms and other debris that have entered the lymph from the interstitial spaces in our tissues.  In addition, lymph nodes are packed full of lymphocytes, which monitor the flow of lymph for foreign substances; thus lymph nodes participate in the activation of the immune system.