The spleen is a fist-sized organ that filters your blood, recycles old blood cells, and helps your immune system fight certain infections. It sits inside your left rib cage, just above your stomach, and despite its small size (roughly that of an avocado in adults), it processes a significant volume of blood every day.
Where the Spleen Sits and How Big It Is
Your spleen tucks behind your lower left ribs, protected by the rib cage. In a healthy adult, it measures less than about 11 centimeters long, 7 centimeters wide, and 5 centimeters thick. Taller people tend to have proportionally larger spleens. A man who is 6 feet tall, for example, can have a spleen up to 14 centimeters long and still be within the normal range.
You typically can’t feel your spleen by pressing on your abdomen. If you or a doctor can feel it below the rib cage, that usually means it’s enlarged, a condition called splenomegaly.
Filtering Old and Damaged Blood Cells
The spleen’s most constant job is quality control for your red blood cells. Red blood cells circulate for about 120 days before they become stiff and worn out. Inside the spleen, blood flows through a mesh of tissue called the red pulp, where cells are forced to squeeze through extremely narrow slits in the walls of blood vessels. Healthy, flexible red blood cells pass through easily. Old, rigid cells get stuck.
Once trapped, these aging cells are broken apart. The spleen’s immune cells then consume the debris and salvage the iron locked inside the hemoglobin. That iron gets shipped back to the bone marrow to build new red blood cells. This recycling system is remarkably efficient: your body recovers and reuses most of the iron from every red blood cell it retires.
The spleen also plays a role in maturing young red blood cells. Newly released red blood cells from the bone marrow still carry leftover bits of internal material. As these young cells squeeze through the spleen’s narrow slits, those unwanted inclusions get physically stripped away. The cell passes through; the debris stays behind. This mechanical “polishing” process transforms immature cells into the smooth, biconcave discs that move efficiently through your bloodstream.
Immune Defense Against Blood-Borne Infections
The spleen contains regions of immune tissue called white pulp, organized much like tiny lymph nodes embedded within the organ. The white pulp has distinct zones: one packed with T cells (the immune cells that coordinate attacks on infected cells) surrounding small arteries, and another dominated by B cells (the immune cells that produce antibodies) clustered in follicles.
What makes the spleen unique among immune organs is how it samples the blood. Unlike lymph nodes, which filter fluid from tissues, the spleen filters blood directly. It has a specialized circulation that slows blood flow down, giving immune cells time to inspect passing particles and pathogens under low-pressure conditions. When B cells in the spleen encounter a threat, they can mature inside structures called germinal centers, producing highly targeted antibodies that get released into the bloodstream.
The spleen is especially important for defending against a specific category of bacteria: encapsulated bacteria, which are coated in a slippery outer shell that makes them hard for other parts of the immune system to grab. The most dangerous of these include the bacteria responsible for pneumococcal infections, meningitis, and certain types of bacterial flu. Without a functioning spleen, the body has a much harder time clearing these organisms from the blood before they multiply out of control.
Blood and Platelet Storage
Beyond filtering and immune defense, the spleen acts as a reservoir. It stores roughly one-third of the body’s platelets, the cell fragments responsible for blood clotting. It also holds a reserve of red blood cells. If you experience sudden blood loss, the spleen can contract and release its stored blood cells into circulation, giving your body a temporary boost while it ramps up production of new cells in the bone marrow.
The Spleen During Fetal Development
Before birth, the spleen has an additional role it later gives up. During the second and third months of pregnancy, the fetal spleen (along with the liver) actively produces red blood cells and platelets. As the fetus develops, the bone marrow gradually takes over this job. By birth, the spleen has transitioned entirely to its filtering and immune roles.
Living Without a Spleen
People can survive without a spleen. Some are born without one, others lose it to injury or surgical removal (splenectomy) due to conditions like severe trauma, certain blood disorders, or cancers affecting the organ. The liver and bone marrow take over much of the blood-filtering work, and lymph nodes compensate for some immune functions.
The real vulnerability is infection. Without a spleen, the risk of a rare but devastating condition called overwhelming post-splenectomy infection rises significantly. This type of sepsis has an annual incidence of about 0.23% in people without a spleen, which sounds small, but it carries a mortality rate of 38 to 70 percent even with treatment. The infections that cause it are almost always from those encapsulated bacteria the spleen specializes in catching.
For this reason, people without a functioning spleen receive vaccinations against pneumococcal disease, meningococcal disease, and Haemophilus influenzae, typically starting during or shortly after their hospital stay. Revaccination against pneumococcal bacteria is recommended every five years. Many people who’ve had a splenectomy also keep a course of emergency antibiotics on hand and wear a medical alert bracelet, because infections that a spleen would normally handle in the background can escalate within hours in its absence.
What an Enlarged Spleen Means
When the spleen swells beyond its normal size, it’s usually responding to something else happening in the body rather than being the source of a problem itself. Common causes include liver disease (which increases blood pressure in the vein feeding the spleen), infections like mononucleosis, blood cancers like lymphoma or leukemia, and certain autoimmune conditions where the spleen is working overtime to destroy blood cells.
An enlarged spleen can cause a dull ache or feeling of fullness in the upper left abdomen, sometimes radiating to the left shoulder. It can also become overactive, trapping and destroying too many healthy blood cells, leading to anemia, frequent infections from low white blood cell counts, or easy bleeding from low platelet counts. In cases of massive enlargement, the spleen becomes vulnerable to rupture from even minor abdominal trauma, which is why doctors often advise avoiding contact sports until the swelling resolves.