While blood types have been around for at least 20 million years, doctors have only known about them for about 116 years. Prior to the 1900s, countless people died when they were injected with the wrong type of blood. This is because blood is composed of proteins called antigens that, when mixed with the wrong type, fight each other as if you're body was fighting an infection. 

Science - Why You Can't Mix Blood Types

science-why-you-cant-mix-blood-types

Let's do English ESL deep listening: focus on meaning. While blood types have been around for at least 20 million years, doctors have only known about them for…