Spleen

The spleen serves multiple functions: as a blood filter, it removes old, damaged, or abnormally shaped red blood cells and destroys pathogens in the bloodstream through resident macrophages. It stores platelets (approximately one-third of the body's platelet reserve) and monocytes. As the largest lymphoid organ, it mounts immune responses to blood-borne antigens by producing antibodies and activating lymphocytes, particularly against encapsulated bacteria.

The spleen is essential for immunity against encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis. These organisms resist phagocytosis without opsonization by antibodies, and the spleen is the primary site for producing IgM antibodies against new antigens in the bloodstream. After splenectomy, patients face lifelong increased risk of overwhelming post-splenectomy infection (OPSI), necessitating vaccinations and sometimes prophylactic antibiotics.

Splenic rupture most commonly results from blunt abdominal trauma such as a motor vehicle accident or sports injury. The spleen's highly vascular parenchyma bleeds profusely when ruptured; the abdominal cavity can accommodate several liters of blood before symptoms become apparent. Splenic rupture causes hemorrhagic shock and requires urgent intervention—either splenectomy or splenic angioembolization in hemodynamically stable patients who wish to preserve the organ.

Splenomegaly is pathological enlargement of the spleen beyond its normal size of approximately 11-13 cm in length. Common causes include portal hypertension from liver cirrhosis (causing congestive splenomegaly), hematologic conditions such as leukemia, lymphoma, hereditary spherocytosis, and sickle cell disease, and infectious diseases including infectious mononucleosis (Epstein-Barr virus), malaria, and visceral leishmaniasis.

The spleen is located in the left hypochondriac region of the abdomen, lying beneath the 9th-11th ribs posterolaterally. It is in contact superiorly with the diaphragm, anteriorly with the stomach (connected by the gastrosplenic ligament), medially with the tail of the pancreas (connected by the splenorenal ligament), and inferiorly with the left kidney. Its position explains why left-lower-rib fractures can lacerate the spleen.