The role of the spleen is to destroy depleted red blood cells from the blood stream, while the marrow is an essential location in the body where blood cells develop. Blood is made in the bone marrow. Once blood is produced, it flows throughout the body.
All cells of the immune system come from bone marrow. Cells form through haematopoiesis; during this process, cells divide into either mature cells of the immune system or into premature cells that travel beyond the bone marrow to continue growing.
The spleen is an essential organ that filters the blood. The spleen consists of T cells, B cells, dendritic cells, macrophages and red blood cells. In addition to eliminating antigens (foreign materials) from the blood, the spleen initiates an immune response to combat infections. The spleen is, essentially, an immunological command center. During times of infection, the spleen's B cells activate and produce massive amounts of antibodies. In addition to these processes, the spleen also destroys old red blood cells.
The roles of the spleen and marrow are essential to understanding the function of the body's immune system. The cells of the immune system are capable of killing parasites, tumor cells, bacteria and virus-infected cells.