The spleen is the organ that filters blood and is part of the immune system. It is found in the upper left quadrant of the human abdomen, and measures approximately 6 to 16 cm. in length in healthy adults. The organ is contained in a thin capsule from which thin fibrous trabeculae enter into the parenchyma. The spleen is composed of red pulp, which has haemocatheretic functions, and white pulp which has immunologic functions. The red pulp consists of a fine meshwork of fibers in continuity with those of the trabeculae, and contain the cords of Billroth and sinusoids in which the blood flows. The white pulp consists of periarteriolar lymphoid sheets (PALS), which represent the T-zone, and of lymphoid follicles, which represent the B-zone.