Although the spleen is not essential for survival, it has important functions such as immune defence, blood filtration, retention and on demand extramedullary hematopoiesis. The white pulp with its T-lymphocytes in perarteriolar lymphoid sheaths and B-lymphocytes in lymphoid follicules is responsible for the detection and removal of circulating pathogens. After splenectomy, this immune defence is missing, which may lead to overwhelming post- splenectomy sepsis by encapsulated bacteria, which has a high mortality rate. In the red pulp, the blood is filtered through narrow slits in the sinusoidal endothelium, which abnormal and senescent erythrocytes can not pass and are eliminated by macrophages. The spleen is, therefore, not a "spleen" of nature, but a sophisticated organ.