BSL Biology Glossary - blood cell - definition
Definition: There are two different types of blood cells: red cells and white cells.
Red cells have something in them called haemoglobin; this makes them rich in iron. The haemoglobin in the red cells pick up the O² (oxygen) from the bloodstream in the air sacs The haemoglobin also gives the red cells their colour.
White cells are different, as they have a nucleus (red cells don't). They are like destroyers. When there are invaders to the body like viruses, bacteria, things causing disease; the white blood cells attack those invaders, eating them up and destroying them. That is their responsibility. So, these blood cells are different and have different jobs; red blood cells and white blood cells.
reproduced by permission from Hodder