Most people are aware of the ABO system and the rhesus system for classifying blood. They're the two most common systems of blood classification, after all.
The ABO system describes blood as type “A,” “B,” “AB” or “O,” and according to the rhesus system, any one of those four types can be further described as “rhesus positive” or “rhesus negative.” But there are many more classification systems that scientists use besides these two, all with their own assortments of blood.
Until recently, researchers recognized 43 separate systems of blood classification. All systems can be used in combination to understand and describe the ways that blood differs from person to person. Now, researchers have identified yet another system, increasing the total count to 44.
