There are three main types of cell in the blood: red cells that carries oxygen, white cells that principally prevent or combat infections, and platelets that help prevent bleeding. There are several types of white cell in the blood. Each cell type is represented in blood in the numbers that meet the functions they serve. One fluid ounce of blood contains about 150 billion red cells, 8 billion platelets, and 20 million white cells. Red cells live for months, platelets for a week or two, and white cells for a few days. The marrow must replace over 200 million cells removed from the blood each day.