Before looking specifically at the condition of pernicious anaemia, it is useful to look at the definitions of the various types of anaemia and understand the basics of vitamin B12 metabolism. Anaemia is defined when there is a reduction of the amount of haemoglobin within the blood, taking into account the person's age and whether they are male or female.
It helps to define anaemia with reference to the size of the red blood cells and the mean corpuscular volume (MCV). Small cells are called microcytes and associated with a low MCV and acommon cause is iron deficiency.Large cells are called macrocytesand they are associated with a high MCV.
Macrocytic anaemia can be further defined by the effect on the bone marrow and further divided into megaloblastic anaemia and non-megaloblastic anaemia. A cause of megaloblastic anaemia is vitamin B12 or folate deficiency and has characteristic signs in the bone marrow; this can be seen when a sample of bone marrow is taken and analysed.