Light micrograph of a section through the spleen. Situated on the left side of the upper abdomen, behind the stomach, the spleen is a large lymphoid organ. It is surrounded by a fibrous capsule (blue, right). The interior consists of nodules of white pulp (not seen) within a matrix of red pulp (pink). The red pulp is responsible for filtering blood by removing any foreign objects or worn out red blood cells. The white pulp contains immune system cells that respond to any pathogens in the blood. Magnification: x100 when printed at 15cm wide.