Specifically, his blood contains an extremely rare enzyme that can be used to treat babies dying of Rhesus disease. If you've never heard of that disease and figure it's not a big deal, well, wait for the numbers.
Harrison, being a generous type, has donated his rare, life-saving blood roughly 1,000 times over 56 years. This has saved the lives of--seriously, you're not going to believe this--over two million babies around the world.
One man (Not JFK) stopped the Cuban Missile Crisis from becoming WW3. One woman's cells have continued to save lives even after her death. These are just a couple of the true heros who have saved billions of lives, often unrecognized, through their actions, good thinking, and altruism. Take James Harrison (pictured):