Light microscopy of a normal human blood smear showing the five types of white blood cells (neutrophil, monocyte, eosinophil, lymphocyte and basophil ). White blood cells or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Red blood cells, also called erythrocytes, form the main cellular component of blood. Millions of erythrocytes in the circulation of vertebrates give the blood its characteristic colour and carry oxygen from the lungs to the tissues. The mature human red blood cell is small, round, and biconcave; it appears dumbbell-shaped in profile. Magnification 450 x when printed at 10 centimetres width.