Light micrograph of human blood cells, showing numerous red cells (erythrocytes) and three neutrophils (white cells). The purple-staining erythrocytes are biconcave discs packed with haemoglobin that act as carriers of oxygen and carbon dioxide to & from tissues. Mature erythrocytes possess no nucleus or other organelles; these are lost during maturity. Neutrophils are the most common type of white cells, acting to engulf invading microorganisms, particularly bacteria. Neutrophils are the main type of white cell involved in acute inflammatory responses. Magnification: x100 at 35mm size.