The human spleen plays diverse roles in the body, with one of the most important being to help filter the blood. In addition to recycling old blood cells, the spleen also acts as a vital center for white blood cells and platelets, and as a combatant of invasive micro-organisms.
The spleen is equipped with a number of passageways that arriving blood must go through. In particular, red blood cells are forced down narrow conduits where healthy cells are allowed through but old or damaged cells detained. In recycling the old cells, the spleen reabsorbs any biomaterial it can for reuse in future cells, especially iron. Additionally, the spleen can act as a reservoir for reserve blood, holding up to a cup's worth at any one time.
In fighting illness, the spleen is partner to the lymphatic system, acting as a co-creator of some of the body's special defensive white blood cells, called lymphocytes. These lymphocytes are critical in containing and stopping infections. The spleen itself is particularly effective in detecting the presence of both bacteria and viruses, and is a first-line defense against such dangerous conditions as pneumonia and meningitis.
Despite the important functions of the spleen, it is possible to live without it, and the surgical removal of all or part of the spleen does occur as a result of certain conditions.