Light micrograph of a section through a lymph node. The fibrous capsule (pink) enclosing the lymph node is seen across top. Lymph nodes are part of the body's immune system. Tissue fluid (lymph) is filtered through the nodes and cellular debris, foreign particles and pathogens are removed. Two secondary follicles (round) are seen within the lymph gland. These contain white blood cells called B lymphocytes. When a pathogen is detected it is presented to these cells, which then make antibodies against the pathogen. Magnification: x100 when printed at 15cm wide.