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Red and white blood cells, conceptual illustration. Red blood cells, or erythrocytes, transport oxygen from the lungs to the rest of the body and also remove carbon dioxide from the body by transporting it to the lungs where it is exhaled. White blood cells are part of the body's immune system and are involved in protecting the body against both infectious diseases and foreign invaders. (KEYSTONE/SCIENCE PHOTO LIBRARY/VIVID BIOLOGY)