Splenic sequestration is a feared complication of sickle cell anemia that primarily affects young children. It is an acute drop in hemoglobin of 2 g/dL accompanied by splenomegaly. The spleen is at particular risk for complications from sickle cell anemia due to its role as a filter of the blood. The spleen is composed of three areas; white pulp, red pulp, and a transitional zone, and each plays a role within the immune system. A minority of blood flow entering the spleen (approximately 10%) is directed outside of the vessels into the parenchyma of the red pulp, where the red blood cells (RBCs) become exposed to reticuloendothelial cells such as macrophages which clear away abnormal cells or other pathogens. To re-enter the vascular system, the RBCs must squeeze through narrow slits in the endothelium of the venous sinuses. The organization of the white pulp of the spleen is predominantly T-cell periarteriolar sheaths and follicles, consisting of B cells. These B cells are critical in the production of IgM antibodies capable of opsonizing encapsulated bacteria such as Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenza type B. Among children with sickle cell anemia, the spleen gradually loses function and size over the first five years of life due to repeated episodes of autoinfarction and scarring. Children with less severe variants of sickle cell disease such as hemoglobin SC disease or sickle cell-beta thalassemia may have preserved some splenic function into adulthood, and studies have shown that treatment with hydroxyurea can preserve and even result in some recovery of splenic function.
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