Acquired aplastic anemia is a rare disorder caused by profound, almost complete bone marrow failure. Bone marrow is the spongy substance found in the center of the long bones of the body. The bone marrow produces specialized cells (hematopoietic stem cells) that grow and eventually develop into red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. In acquired aplastic anemia, an almost complete absence of hematopoietic stem cells eventually results in low levels of red and white blood cells and platelets (pancytopenia). Specific symptoms associated with acquired aplastic anemia may vary, but include fatigue, chronic infections, dizziness, weakness, headaches, and episodes of excessive bleeding. Although some cases of acquired aplastic anemia occur secondary to other disorders, researchers now believe that most cases result from a disorder of the patient's immune system, in which the immune system mistakenly targets the bone marrow (autoimmunity). This is based on the response of approximately half of patients to immunotherapy, whether it is ATG, cyclosporine, high-dose steroids or cyclophosphamide.