Lymph node. Coloured transmission electron micrograph (TEM) of a section through a lymph node to show the lumen of a medullary sinus containing a variety of lymphocytes (yellow). Lymphocytes are involved in the immune system's defence mechanisms, lymph system, and antibody production. These cells will migrate to blood vessels and circulate around the body when stimulate by the immune response. Magnification: x3000 when printed 10 centimetres wide.