When the body does not have a sufficient supply of vitamin B12, macrocytes (or megaloblasts) or abnormally large red blood cells are produced by the body. Sometimes the size of the abnormal cells does not allow them to leave the bone marrow and travel into the bloodstream the way normal red blood cells do. This means there are fewer cells in the bloodstream capable of carrying oxygen to the various tissues around the body. This is due to the irregularly large cells being produced instead of normal red blood cells.
When an individual has these abnormally large cells in the blood, it is called macrocytic anemia. Someone who has macrocytic anemia has more of a chance of having pernicious anemia than others, simply because pernicious anemia is a subtype of macrocytic anemia. Usually, when pernicious anemia is associated with megaloblasts, symptoms manifest as general weakness, excessive fatigue, and lightheadedness. These symptoms occur from the lack of oxygen supply to the body tissues due to the presence of too many megaloblasts.