Until recently, blood was simply blood. But in recent years, we have discovered different blood types and that can literally save our lives when we need transfusions. All blood can be categorised as Rhesus positive or negative, but why this should be the case, we have no idea. We also know nothing about why human beings have four blood types but cows have 800. Read this article to find out more about the mysteries of our blood (900 words).
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