The spleen, a fist-sized, dark red organ, is situated in the upper left side of the abdomen, beneath the rib cage. Weighing approximately 150 to 200 grams in a healthy adult, it functions as the largest secondary lymphoid organ in the human body. This organ manages both the quality of the blood and the body’s systemic immune response against bloodborne threats. It continually filters the entire blood volume, removing cellular debris and initiating defenses against pathogens that enter the circulation.
The Spleen’s Specialized Immune Architecture
The spleen’s complex functions are made possible by its unique internal structure, which is divided into two primary zones: the red pulp and the white pulp. The red pulp constitutes the majority of the splenic tissue and appears red due to its high concentration of red blood cells and blood-filled cavities. Its architecture is designed for processing and filtering the circulating blood.
The white pulp, which makes up less than a quarter of the splenic tissue, is the organ’s dedicated immunological center. It is organized into areas rich with T lymphocytes and B lymphocytes, the main cells of the adaptive immune system. The interface between these two major zones, known as the marginal zone, serves as a specialized screening area for antigens entering from the bloodstream.
Primary Defense: Blood Filtration and Innate Response
The red pulp is the body’s largest blood-cleaning station, performing a specialized function called culling and pitting. As blood flows through the red pulp’s intricate network of splenic cords and venous sinuses, it undergoes a rigorous quality control process. Old, damaged, or abnormally shaped red blood cells are unable to squeeze through the narrow openings of the venous sinuses and are trapped.
These trapped cells are then engulfed and destroyed by specialized phagocytic cells, primarily red pulp macrophages. This process removes worn-out cells, preventing them from releasing harmful contents into the bloodstream, and allows for the recycling of iron back into the body. The dense population of macrophages also provides an immediate, non-specific innate immune response by trapping and eliminating foreign debris, immune complexes, and bloodborne pathogens.
The spleen is uniquely equipped to clear bacteria that possess a polysaccharide capsule, such as Streptococcus pneumoniae and Haemophilus influenzae. This capsule helps the bacteria evade phagocytosis by other immune cells. Splenic macrophages, assisted by locally produced antibodies and complement proteins, destroy these encapsulated bacteria from the circulation. This clearance mechanism safeguards against rapid-onset systemic infections, or sepsis, originating in the blood.
Coordinating Adaptive Immunity
The white pulp is responsible for initiating the systemic adaptive immune response, which provides specific, long-lasting immunity against pathogens. Bloodborne antigens, or foreign molecules, are first captured by specialized antigen-presenting cells, such as dendritic cells and marginal zone B cells, as they enter the white pulp. These cells then migrate to the T-cell and B-cell zones, where they present the captured antigens to circulating lymphocytes.
The T-cell zones, known as the periarteriolar lymphoid sheaths, are where helper T cells are activated upon recognizing the presented antigen. Activated helper T cells collaborate with B cells in the surrounding lymphoid follicles to initiate a humoral immune response. This collaboration drives the B cells to proliferate and differentiate into plasma cells, which produce large quantities of highly specific antibodies.
This process results in specific immune memory against the detected pathogen. The spleen is important for generating the IgM memory B cells that circulate in the marginal zone, which are crucial for a rapid antibody response upon re-exposure to bloodborne antigens. The production of these antibodies coats the invading pathogens, a process called opsonization, enhancing their removal by innate macrophages in the red pulp.
Immune Life After Spleen Removal
The surgical removal of the spleen, known as a splenectomy, or conditions causing the spleen to malfunction, results in a state called asplenia or hyposplenism. While the body can survive without the organ, the absence of the spleen’s filtering and immune-activating functions leads to a permanent vulnerability to certain infections. The most significant risk is an overwhelming post-splenectomy infection (OPSI), a severe and rapidly progressing form of sepsis.
This risk is due to the impaired ability to clear encapsulated bacteria. Without the splenic macrophages and specialized IgM memory B cells, pathogens like Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae multiply unchecked in the bloodstream. Individuals without a functional spleen require specific precautions, including a strict schedule of vaccinations against these bacteria. They are also advised to carry a course of broad-spectrum antibiotics to be started immediately at the first sign of fever or infection, as the infection can progress quickly.