Abstract Red blood cells (RBCs) carry oxygen bound reversibly to the ferrous Fe ++ atoms of the four haem groups of the haemoglobin (Hb) tetramer. In order to transport the Hb around the body in a functional state, the RBC requires a flexible membrane and contents to pass passively through the capillary bed and a source of energy to maintain the internal milieu. ATP is provided by anaerobic glycolysis. Reducing power is provided as NADH and NADPH, via the pentose phosphate pathway. Genetic abnormalities that affect the membrane deformability lead to shape changes and haemolysis; defects in the glycolytic pathway cause non-spheroctic haemolytic anaemia and failure of reducing power to intravascular haemolysis in response to oxidative stress. White blood cells provide the basis for the innate immune system as well as interacting with specific immune processes. They need to pass from the circulation, through the vessel wall into the extravascular tissues in order to carry out these functions. Inherited defects of the migratory process also lead to susceptibility to infection.