The life span of white blood cells is from hours to years, depending on the body's needs. If you have an infection, lymphocytes kill the germs (or at least they try) and some of them secrete antibodies that attack the germs. So in an infection, the number of white blood cells increases and they die faster. Some of them remember that the germ was in your body and the remain in your blood stream for years so if the germ appears again, they attack it with much more efficiency. Red blood cells do not live any longer than 120 days.
platelets circulate for about 9 days. If they encounter damaged blood vessel walls during this time, they stick to the damaged area and are activated to form a blood clot. This plugs the hole. Otherwise, at the end of their life span they are removed from the circulation by the spleen.
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