‘What is blood?’ describes the constituent parts of blood: a fluid called plasma in which three different cell types are carried—red cells, white cells, and platelets. Red cells are dominated by the red protein haemoglobin that plays the key role in transporting oxygen from the lungs to the tissue. A lack of red blood cells is called anaemia. A lack of iron is the main cause, iron being required to make a functional haemoglobin protein. White cells, or leucocytes, perform many and varied roles in the body, but all are related to immune defence against pathogens. Platelets have only one function: the prevention of excess bleeding through coagulation.