congenital blood disorder (blood disease) characterized by an enlarged spleen, spherical (rather than disk-shaped) red blood cells (erythrocyte) of variable size and increased fragility of cell membrane, and a chronic, mild hemolytic anemia punctuated by episodes of severe aplastic anemia (failure of bone marrow to produce cells). Hereditary spherocytosis, the most frequent of the familial anemias, is inherited as an autosomal dominant trait; it is most common among people of northern European descent. Symptoms may be present at birth or may not appear until old age; the disease is most often identified during childhood or adolescence. Removal of the spleen results in cure of anemia in nearly all cases; the defects in blood production remain but do not cause serious disability.