The Role of the Spleen in Vertebrates
The spleen is an essential organ found in all vertebrates, functioning similarly to a large lymph node. Its primary role is to act as a filter for the blood, playing a crucial role in both the circulatory and immune systems. One of the spleen's main tasks is the removal of old red blood cells, a process that allows the body to recycle blood components, ensuring the availability of fresh blood, especially in cases of hemorrhage. Additionally, the spleen enables the recycling of iron from red blood cells, which is essential for various bodily functions. Old red blood cells are broken down by the mononuclear phagocyte system, with hemoglobin being split into globin and heme. Heme is further processed in the liver into bilirubin, facilitating recycling. The spleen also plays a central role in the activities of the mononuclear phagocyte system and can be likened to a large lymph node. Its red pulp is home to more than half of the body’s monocytes, which move to tissue injury sites, aiding in regeneration by differentiating into dendritic cells or macrophages. In its white pulp, antibodies are produced, helping to eliminate bacteria or damaged cells.
Anatomy and Function of the Spleen
Structure and Immune Response
The spleen is a large, oval-shaped secondary lymphoid organ located in the upper left part of the abdomen. Its primary function is to mediate immune responses against blood-borne antigens. While lymph nodes primarily respond to antigens derived from surrounding tissues, the spleen filters the blood, capturing and responding to antigens within it. This makes the spleen highly specialized in defending against systemic infections. Unlike lymph nodes that receive antigens via lymphatic vessels, the spleen receives antigens through the splenic artery. Studies have shown that a significant volume of blood, along with many antigens, passes through the spleen each day, more so than through all the body's lymph nodes combined.
Spleen Structure
The spleen is encased in a capsule from which several projections, called trabeculae, extend inward, providing structural support. Internally, it consists of two distinct areas: the red pulp and the white pulp. The red pulp is filled with capillaries, macrophages, a large number of red blood cells, and a small number of lymphocytes. This region is where old or defective red blood cells are destroyed and removed. The macrophages in the red pulp often capture red blood cells and contain pigments derived from iron that was part of the hemoglobin. The white pulp surrounds the blood vessels entering the spleen and is primarily composed of the periarteriolar lymphoid sheath (PALS), which is rich in T lymphocytes. Attached to PALS are primary lymphoid follicles, which are abundant in B lymphocytes. At the edge of the PALS, there is a diverse mix of lymphocytes and macrophages.
Immune Activation in the Spleen
Antigens and lymphocytes enter the spleen through the splenic artery, eventually reaching the PALS. Here, dendritic cells capture antigens and begin the process of presenting them to T lymphocytes. This triggers the activation of B and T lymphocytes, starting with dendritic cells displaying antigen fragments on MHC II molecules to helper T cells. Once activated, helper T cells stimulate B cells to move into the lymphoid follicles at the PALS margin. When the antigen stimulus is sustained, these B cells rapidly divide and differentiate, forming a secondary lymphoid follicle.
The Impact of Spleen Loss
Losing the spleen, through splenectomy or other means, increases susceptibility to various infections. Children who undergo splenectomy are particularly vulnerable to sepsis caused by bacterial infections. While adults who lose their spleen may face less severe consequences, they are still at an elevated risk for bacterial bloodstream infections.
Additional Functions of the Spleen
In addition to its primary roles, the spleen has other functions that are less prominent in healthy adults but become vital under certain conditions.
Hematopoiesis in Fetal Development
During fetal development, the spleen is responsible for producing all types of blood cells. Although this function diminishes after birth, with hematopoiesis shifting primarily to the bone marrow, the spleen still plays a significant role in circulating and storing blood cells in the immune and circulatory systems.
Blood and Cell Storage
The spleen acts as a reservoir for red blood cells, lymphocytes, and platelets. In horses, for instance, up to 30% of red blood cells can be stored in the spleen. In humans, it is estimated that the spleen can hold about 240 milliliters of red blood cells, releasing them into the bloodstream in response to conditions like hypovolemia or hypoxia. It also stores platelets for emergencies and removes old or damaged ones from circulation. Around a quarter of the body's lymphocytes may also be stored in the spleen.
Related Terminology
Spleen, red blood cells, lymphocytes, white pulp, red pulp
References
Kuby Immunology (7th ed.) (2013)
0 Comments