Iron is an important mineral for the blood, and it is often cited as a treatment of anemia. Iron is a part of hemoglobin, the protein that binds oxygen in red blood cells. So a lack of iron means a lack of hemoglobin, and a lack of hemoglobin means a lack of red blood cells.
But also, there are proteins produced by the body that stimulate red blood cell creation in the bone marrow (that's where red blood cells are made!). Some treatments for anemia, like the brand Procrit (injectable synthetic epoietin alfa) mimics erythropoietin, turning on the erythrocyte machinery. However, there are side effects to treatment - as the number of cells in the blood is increased, the blood volume increases, which could increase blood pressure. Clotting is also an issue.
The reason for the anemia would determine what treatment - whether it be with iron supplementation or synthetic erythropoietin, or any other stimulant of red blood cell creation or prevented destruction - will depend on its cause.
What about white blood cells? Read about that tomorrow.