The spleen serves two major functions in humans. First, it plays an important role in adaptive immunity; groups of B and T lymphocytes reside in the spleen. These white blood cells produce antibodies, fight bacteria and viruses and destroy infected cells. Second, the spleen holds a third of the body's platelets in reserve. If a person suffers an internal hemorrhage, the spleen releases these platelets into the circulation.
The spleen also plays a role in removing and recycling senescent or damaged red blood cells; however, the bulk of this activity takes place in the liver.
The spleen is located so deep inside the body that it can't be palpated in the vast majority of healthy people. In certain disease states, however, the spleen becomes enlarged; this condition is known as splenomegaly. Certain viral infections, such as Epstein-Barr Virus, as well as leukemia or lymphoma can cause splenomegaly. An enlarged spleen is prone to rupture; a ruptured spleen is a life-threatening event that can only be treated surgically.
In people with sickle cell anemia, the spleen sustains progressive damage starting in infancy. By adolescence, sickle cell patients have no functional spleen left. Although functional asplenia raises the risk of infections, in particular streptococcal pneumonia, meningococcus, salmonella and Haemophilus influenzae, vaccinations against these encapsulated bacteria are generally effective at protecting sickle cell patients.