Aplastic anemia (AA) is a disease in which the bone marrow, and the blood stem cells that reside there, are damaged. This causes a deficiency of all three blood cell types (pancytopenia): red blood cells (anemia), white blood cells (leukopenia), and platelets (thrombocytopenia). Aplastic refers to inability of the stem cells to generate the mature blood cells. It is most prevalent in people in their teens and twenties, but is also common among the elderly.