Triggered by a lack of substances necessary for hematopoiesis.

For example, iron and protein are needed to make red blood cells, and vitamins are needed to mature red blood cells. Iron deficiency is very common due to malnutrition, and iron deficiency can cause iron deficiency anemia. If there is a lack of vitamin B12 and folic acid, immature red blood cells will not be able to synthesize DNA, causing the red blood cells to fail to mature, causing megaloblastic anemia