Pernicious anaemia. Light micrograph of megaloblast blood cells from bone marrow in a case of pernicious anaemia, also known as Biermer's anaemia. A megaloblast is an enlarged erythroblast, the immediate precursor to a mature red blood cell. Pernicious anaemia is caused by a deficiency of vitamin B12 or folic acid due to the intestine being unable to absorb these substances. Compared to a normal blood smear, the number of red blood cells is severely reduced and those that are present are enlarged or deformed. Symptoms of the condition include tiredness and lowered immunity.