What causes stress reticulocytes?

Anemia: Reticulocytes are normal immature red blood cells. Their number in the blood stream becomes increased when the body senses that it does not have enough red blood cells (anemia), so the bone marrow tries to make more red blood cells. Normally reticulocytes reside in the bone marrow, but they escape into the blood. This can happen during anemia caused by hemolysis (red blood cell destruction).