Blood, smear. Light microscopy of a normal human blood smear showing four neutrophils. Neutrophil granulocytes (also known as neutrophils) are the most abundant (40% to 75%) type of white blood cells in mammals and form an essential part of the innate immune system. They are formed from stem cells in the bone marrow. They are short-lived and highly motile. 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. The cell is flexible and assumes a bell shape as it passes through extremely small blood vessels. Magnification 300 x when printed at 10 centimetres width.