Anemia is a term for a family of diseases that give low red blood cell counts or low hemoglobin or iron levels in blood. Either way, since iron and hemoglobin in red blood cells are responsible for the transport of oxygen throughout the body, these diseases have serious health consequences.
Red blood cells are also called erythrocytes. Iron deficiency is a form of anemia that results from either lack of absorption of dietary iron or loss of iron through internal bleeding issues. Diagnosis of this and other types of anemias include obtaining red blood cell counts and hemoglobin concentrations. But another aspect to the diagnosis includes looking at erythrocytes under a microscope. Iron deficiency will show cells that are small or with variation in shape and size. Most tellingly, pale colored cells will be seen. Since the iron ion in hemoglobin is responsible for the red color in blood, pale colored red blood cells is a major red flag for iron deficiency.
Pernicious anemia results in low production of red blood cells. Cells under a microscope in this case will show unusually large red blood cells called megaloblasts. Another problem associated with pernicious anemia is the lack of vitamin B12 absorption by the body. Since B12 is not produced in the body and must be absorbed from food, vitamin B12 levels are also monitored and tested in cases of pernicious anemia.
In both cases, I do not believe that other blood cell types are affected.