Because the spleen filters blood as the lymph nodes filter lymph, it functions in both the immune and the hemopoietic systems. Immune system functions of the spleen include  antigen presentation by APCs (mostly dendritic cells and macrophages) and initiation of immune response;  activation and proliferation of B and T lymphocytes;  production of antibodies against antigen present in circu lating blood; and removal of macromolecular antigens from the blood.
Activation and proliferation of T cells and differentiation of B cells and plasma cells, as well as secretion of antibodies, occur in the white pulp of the spleen; in this regard, the white pulp is the equivalent of other lymphatic organs.
Hemopoietic functions of the spleen include:  removal and destruction of senescent, damaged, and abnormal erythrocytes and platelets;  retrieval of iron from erythrocyte hemoglobin;  formation of erythrocytes during early fetal life; and  storage of blood, especially red blood cells, in some species.
The role of the red pulp is primarily blood filtration (i.e., removal of particulate material, macromolecular antigens, and aged, abnormal, or damaged blood cells and platelets from the circulating blood). These functions are accomplished by the macrophages embedded in the reticular mesh-work of the red pulp. Senescent, damaged, or abnormal red cells are broken down by the lysosomes of the macrophages; the iron of the hemoglobin is retrieved and stored as ferritin or hemosiderin for future recycling. The heme portion of the molecule is broken down to bilirubin, which is transported to the liver via the portal system and there conjugated to glucuronic acid. Conjugated bilirubin is secreted into the bile, giving it a characteristic color.
Macrophages recognize senescent or abnormal blood cells by several different mechanisms: become more rigid and are thus more easily trapped in the mesh of the red pulp.
Specific mechanisms include opsonization of the cell membrane with anti-band 3 IgG antibodies, which trigger Fc receptor–dependent phagocytosis of erythrocytes. In addition, specific changes in glycosylation of glycophorins (see page 274) in aging erythrocytes act as a recognition signal that triggers the elimination of senescent erythrocytes by macrophages.
Despite these important functions, the spleen is not essential for human life. It can be removed surgically, which is often done after trauma that causes intractable bleeding from the spleen. The removal and destruction of aging red blood cells then occurs in the bone marrow and liver.
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