Spleen – Structure and Function
The spleen is located in the left hypochondriac region of the abdomen between the fundus of the stomach and diaphragm. In adults, it is approximately 12 cm long, 7 cm wide, and 3 cm in thickness, and weighs around 150-250 g. The splenic artery, splenic vein, efferent lymphatic vessels and splenic nerve plexus pass through the hilus, which is a depressed area in the capsule.1, 2, 3
The spleen is the largest organ of the lymphatic system and contributes to a fully operational immune system as it combines the innate and adaptive immune system in a uniquely organized way. The structure of the spleen enables it to remove older red blood cells from circulation and leads to the efficient removal of blood-borne microorganisms and cellular debris. This function, in combination with a highly organized lymphoid compartment, makes the spleen the most important organ for antibacterial and antifungal immune reactivity.
The spleen is an organ which not only effectively uses its own immune cells but also mobilizes the body’s immune cells for immune surveillance and for the protection of other vital organs including the heart, kidney and brain.4, 5, 6, 7 The spleen is prone to physical injury, infections, and various immunological conditions including cancers. Enlargement of the spleen or splenomegaly may occur due to anemia, infections, inflammation, cancer, metabolic disorders, and liver diseases.
The spleen has four important structures:
and marginal zone.
Each area shows unique morphological structure and is involved in performing specific physiological functions. The capsule contains dense connective tissues, elastic and smooth muscle fibers, and sympathetic nerve fibers from the splenic nerve plexus. The cells which play an important role in spleen functions are macrophages, monocytes, natural killer (NK) cells, and B- and T-cells.
Red pulp constitutes about 70% of the total splenic volume in adults and contains numerous sinuses which are filled with blood, rich in platelets. Between the sinuses are spongy cellular cords (cords of Billroth), made up of reticular fibers and reticular cells intermingled with several immune cells, such as macrophages, monocytes, granulocytes, B-cells, T-cells and plasma cells. In the red pulp, pathogens and cellular debris, as well as aging erythrocytes, are efficiently removed from the blood by macrophages, which are abundant in this compartment. These macrophages are then well equipped to recycle iron from the erythrocytes. Several red pulp specific functions occur in the spleen including blood filtration, antigenic stimulation and proliferation of B- and T-cells and production of antibodies of different specifications.
The Marginal Zone
The marginal zone forms a bridge between the innate and adaptive immune response, because the macrophages in this region, which express specific pattern-recognition receptors, can efficiently take up blood-borne pathogens. The specific subset of B-cells in this region, the marginal-zone B-cells, can be activated by these macrophages or can directly respond to blood-borne pathogens, after which they become antigen-presenting cells or IgM-producing plasma cells. Entry of activated dendritic cells or marginal-zone B-cells to the white pulp can initiate an adaptive immune response through activation of T-cells, which then migrate to the edge of the B-cell follicles and provide help to B-cells.
Immunological activation of B-cells occurs in the marginal zone as a result of antigenic encounter.8 Many lymphocytes in the marginal zone migrate into respective T- and B areas. The marginal zone contains the highest concentration of blood antigens of any other area in the spleen because splenic arterial blood empties into the marginal zone. Marginal zone B-cells show somatic hypermutation, clonal expansi