The spleen is a soft organ nestled in the upper left abdomen and is a specialized component of the body’s defense system. It functions as the main filter for the blood, constantly surveying the circulation for foreign invaders and damaged cells. It is often described as the “lymph node of the blood” because it is the primary site where immune responses are launched against pathogens that have entered the bloodstream.
Internal Organization of the Spleen
The spleen’s structure is organized to support its dual functions of immunity and filtration, separating into two distinct types of tissue: the white pulp and the red pulp. The white pulp constitutes the immunologic center, consisting primarily of clusters of lymphocytes arranged around arterioles, similar to the architecture found in lymph nodes. This area is designed for immune surveillance, housing T cells and B cells ready to encounter antigens.
The red pulp makes up the majority of the spleen’s mass and handles blood filtration and recycling. It is composed of splenic cords, known as the Cords of Billroth, and wide vascular channels called sinusoids, which are packed with specialized macrophages. Sandwiched between these two regions is the marginal zone, a border area where bloodborne antigens first encounter resident immune cells. This location makes the marginal zone the initial checkpoint for all materials entering the spleen from the circulation.
Initiating Adaptive Immune Responses
The marginal zone plays a role in capturing pathogens and initiating an adaptive immune response against bloodborne threats. Specialized marginal zone B cells and macrophages intercept antigens flowing from the blood and process them for presentation. These cells then ferry the antigenic information into the adjacent white pulp, the staging ground for T and B lymphocytes.
Within the white pulp, T lymphocytes are concentrated in the periarteriolar lymphoid sheath (PALS), which surrounds the central arteriole. Antigen-presenting cells, such as dendritic cells, migrate here to display fragments of captured pathogens to the T cells. This interaction activates specific T cells, which then proliferate and coordinate the broader immune attack.
Adjacent to the T cell zones are B cell follicles, where B lymphocytes reside awaiting activation. Once a T cell is activated, it provides necessary signals to a corresponding B cell that has also recognized the same antigen. This interaction triggers the B cell to multiply and differentiate into plasma cells, which are antibody factories.
The activated B cells often form germinal centers within the follicles, which are sites of intense proliferation and mutation, refining the quality of the antibodies being produced. Plasma cells then release large quantities of specific antibodies into the bloodstream, which neutralize the circulating pathogen. The white pulp’s ability to mobilize this humoral response against systemic infection is important for defense against encapsulated bacteria.
Blood Filtration and Cellular Recycling
Beyond its role in adaptive immunity, the red pulp performs the task of managing the body’s blood cells. This function is carried out by a dense network of macrophages that line the splenic cords and sinusoids. As blood flows through this area, it is subjected to a quality control process.
Old or damaged red blood cells (RBCs) must squeeze through narrow gaps, called interendothelial slits, to pass from the cords into the venous sinusoids. Since aged RBCs become less flexible after their typical 120-day lifespan, they fail this test and are subsequently destroyed by the waiting macrophages, a process known as culling. This mechanism ensures that only healthy, flexible cells remain in circulation.
Another process performed by the red pulp macrophages is called pitting, where they selectively remove abnormalities or inclusions from the surface of circulating RBCs. These inclusions, such as Howell-Jolly bodies, are “pitted” out of the cell without destroying the entire RBC. The red pulp also acts as a reservoir, holding platelets and monocytes that can be deployed into the blood during injury or infection.
Life Without a Spleen
The removal of the spleen (splenectomy) or its functional failure significantly alters the body’s ability to fight certain infections. Without the spleen, the body loses its primary site for filtering the blood and initiating an immune response against bloodborne pathogens. This loss results in a heightened susceptibility to overwhelming post-splenectomy infection (OPSI).
The risk is high for infections caused by encapsulated bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis. These bacteria are difficult for the immune system to clear because their polysaccharide capsules prevent effective phagocytosis. The spleen is normally the main organ producing the quick antibody response needed to overcome this defense. To mitigate this risk, patients without a spleen must receive specific prophylactic vaccinations, including the pneumococcal, meningococcal, and Hib vaccines, often with periodic boosters. Some individuals may also be advised to take daily low-dose antibiotics for additional protection against these severe infections.