The spleen performs complex functions fundamental to maintaining blood health and immune defense. Located in the upper left section of the abdomen, underneath the rib cage, this small, purple organ is roughly the size of a person’s fist in an adult human. The spleen is classified as the largest organ of the lymphatic system. Its unique, highly vascular structure allows it to act as a specialized blood filter, providing a constant quality check on the circulation. This organ plays an important role in the body’s defenses and in the recycling of cellular components.
Immune Surveillance and Defense
The spleen contains a specialized area known as the white pulp, which is dedicated to immunological function. This tissue is structurally similar to a large lymph node but is uniquely positioned to screen blood-borne pathogens rather than tissue fluid. The white pulp consists of lymphoid follicles rich in B-lymphocytes and periarteriolar sheaths dominated by T-lymphocytes and macrophages. When blood flows through the spleen, these immune cells are strategically positioned to detect antigens and foreign invaders.
This system is particularly important for initiating an immune response against encapsulated bacteria, such as Streptococcus pneumoniae and Neisseria meningitidis. These bacteria possess smooth polysaccharide capsules that make them difficult for general immune cells to engulf. The B-lymphocytes within the white pulp are activated upon encountering these pathogens and subsequently mature into plasma cells to produce antibodies, especially IgM. These antibodies then coat, or opsonize, the bacteria, making them recognizable targets for destruction by macrophages.
Blood Filtration and Cellular Recycling
The red pulp focuses on maintaining the quality of circulating blood. The red pulp is a meshwork of blood-filled venous sinuses and splenic cords, which are rich in specialized macrophages. As red blood cells (RBCs) flow through this area, they must squeeze through narrow slits in the walls of the venous sinuses to re-enter the bloodstream. This process serves as a rigorous mechanical filter for blood quality control.
Old, damaged, or inflexible red blood cells are unable to pass through these tight passages. These trapped cells are retained within the splenic cords. Once retained, they are engulfed and destroyed by the resident macrophages. The spleen is also involved in “pitting,” which is the removal of abnormal inclusions, like nuclear remnants or excess membrane, from an otherwise salvageable red blood cell without destroying the entire cell.
The destruction of aged red blood cells allows for the efficient recycling of their components. The hemoglobin within the destroyed cells is broken down, and the valuable iron atom is salvaged. This salvaged iron is then transported back to the bone marrow to be reused in the creation of new hemoglobin and, ultimately, new red blood cells. The remnants of the hemoglobin protein are degraded into amino acids, which are also reused by the body.
Spleen as a Blood Reservoir
The spleen functions as a storage site for various blood components that can be rapidly mobilized when the body requires them. The red pulp’s spongy structure allows it to hold a reserve of blood volume, which can be released into the general circulation during times of physiological stress.
In humans, the spleen sequesters a significant portion of the body’s platelets, which are cell fragments necessary for blood clotting. It can hold approximately 25% of the total platelet count. When the body experiences trauma or sudden blood loss, the spleen can contract its fibroelastic capsule and trabeculae. This contraction rapidly injects the stored platelets and some red blood cells into the bloodstream, helping to stabilize blood volume and contribute to hemostasis.
Life Without the Spleen
When the spleen is surgically removed, a person can continue to live a relatively normal life because other organs compensate for its absence. The liver and bone marrow take over the primary responsibility for clearing old or damaged red blood cells and recycling the iron. However, the absence of the spleen results in a permanent and lifelong vulnerability to certain types of severe bacterial infections.
The resulting condition, overwhelming post-splenectomy infection (OPSI), is a rare but rapidly progressing and potentially fatal form of sepsis. To manage this increased risk, individuals without a spleen must receive prophylactic vaccinations against common encapsulated organisms, specifically Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. They are also often advised to carry a course of emergency antibiotics.