Aplastic anemia is a condition in which the quantity of bone marrow is reduced and the ability to produce the blood cells, therefore is below normal. There are degrees of aplastic anemia but in severe aplastic anemia, the bone marrow is virtually empty of blood-forming cells and with this condition, therefore, are at a high risk for infection and bleeding and require transfusions to maintain their blood. In less severe cases, the bone marrow quantity may be reduced, but to a lesser degree and the blood count may be reduced, but also to a degree that is not as life threatening. In the majority of cases of aplastic anemia, the cause is unknown. It can occur after exposure to certain toxins and certain drugs, which can damage the bone marrow. It can be seen after certain viral infections, most particular HEPATITIS virus, and would represent a very serious complication of that infection. In some cases, withdrawal of the drug results in recovery, but in other cases, when the damage is severe, that is not the case

In a significant percentage of patients with aplastic anemia, particularly when the cause is unknown, the mechanism appears to be immunologic. That is, some immune protein that the body makes is actually attacking the marrow stem cells and causing them to disappear. Therefore a treatment to suppress the imune system can allow the bone marrow to grow back and produce a remission, either partial or complete. The most common drug used is antithyocite globulin, which targets T-lymphocytes. Such drugs as CYCLOSPORINE, used to prevent graft rejection, also have efficacy.

In patients who fail to respond to this kind of immune suppressive therapy, and who are young enough to withstand bone marrow transplant from a donor who is compatible can result in recovery. Bone marrow transplant can represent a cure in a significant percentage of patients, though only a limited number of patients will have compatible donors. Patients who have severe aplastic anemia, cannot have transplants and do not respond to immunosuppressive medications and overall survival tends to be quite short.