Light micrograph of a section through a lymph node. 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. A secondary follicle (round) is seen at top centre. It contains 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: x200 when printed at 15cm wide.