5.1 The two major diseases of the red blood cell are anaemia (where there are insufficient red blood cells and/or haemoglobin) and erythrocytosis/polycythaemia (where there is an excess of red blood cells and/or haemoglobin).
5.2 Anaemia can be classified according to the size of the red blood cell into microcytic (small red blood cells: a low MCV), normocytic (the red blood cell size is within the reference range: normal MCV), and macrocytic (where red blood cells are large: a high MCV).
5.3 The two most common forms of congenital reduction in the number of red blood cells are Diamond–Blackfan anaemia and Fanconi’s anaemia.
5.4 The principal types of haematological cancer that can lead to anaemia are myelofibrosis, leukaemia, myeloma, lymphoma, and myelodysplasia.
5.5 The principal causes of iron deficiency are insufficient iron in the diet, poor absorption across the gut wall, and reduced ability to transport iron in the blood to the bone marrow.
5.6 The principal causes of excess iron are increased ingestion of iron in the diet, a high frequency of transfusion, and increased absorption across the intestine.
5.7 Ring sideroblasts (nucleated red cells containing siderosomes) are found in the bone marrow and are indicative of the failure to incorporate iron into haem. They are therefore a sign of defective erythropoiesis at the level of the mitochondrion.
5.8 The stomach is important for the absorption of vitamin B12 because it provides an acid environment and enzymes to dissociate the vitamin from foodstuff, and intrinsic factor to enable the absorption of the vitamin in the lower intestines.
5.9 The primary causes of vitamin B12 deficiency are malnutrition and malabsorption. The major cause of the latter is disease of the stomach. In turn, the major disease of the stomach is autoimmune gastritis which, if related to autoantibodies, defines pernicious anaemia.
5.10 The primary FBC, blood film, and bone marrow abnormalities to be expected in vitamin B12 deficiency are, respectively, low haemoglobin and macrocytes; hypersegmented neutrophils; giant metamyelocytes and megaloblasts.
5.11 The pathological basis of anaemia of chronic diseases depends on the particular aetiology—in inflammatory disease it may be suppression of erythropoiesis and hypoferraemia whilst in some cancers it may be invasion of the bone marrow by tumour. However, whatever the aetiology, the anaemia is generally normocytic.