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 (RBCs) is severely reduced and those that are present are enlarged or deformed. Symptoms of the condition include tiredness and lowered immunity.