decrease in number of red blood cells

Anemia (U.S. spelling) or anaemia is not having enough red blood cells or hemoglobin in the blood.

Blood film with iron deficiency anemia.
Sickle cell anemia. Normal red blood cells are shown in Figure 1; with sickle cell anemia, red blood cells look like figure 2

Types or causes of anemia change

Microcytic anemia change

Normocytic anemia change

Macrocytic anemia change

Dimorphic anemia change

Dimorphic anemia means two types of anemia at the same time. For example, macrocytic hypochromic, due to hookworm infestation leading to not enough of both iron and vitamin B12 or folic acid or following a blood transfusion.