The spleen is a unique organ, positioned in the abdomen underneath the diaphragm the spleen is the biggest secondary lymphoid organ. It has two functionally distinct zones. The red pulp and the white pulp, and the boundary between the two the marginal zone. The red pulp is mainly a venous sinous. The white pulp is the spleen lymphoid part, it is composed from the T cells area arranged around a central arteriole and named PALS (periarteriolar lymphoid sheath) and the surrounding B cells area or follicles. The red pulp is the body blood filter system, it digests and recycles blood cells by the red pulp macrophages population. The white pulp is the center for adaptive immune response generation, whereas the marginal zone populated with macrophage
