Daniel wants to know what blood type he is.

Mom tells him. Then he wants to know what that actually means — and why we don't all just have the same blood.

The answer starts with tiny markers on the surface of red blood cells — think of them like little flags. Your immune system learns to recognize your own flags as safe. Everything else it treats as a threat. If you're type A, your body makes weapons against type B. If you're type B, it makes weapons against type A. If you're type O — it makes weapons against both.

Which means if you ever get the wrong blood type in a transfusion, your immune system attacks it immediately.

That's exactly what used to happen — before anyone understood why.

Before 1901, the assumption was that all human blood was basically the same. When transfusions went wrong and patients died, docto ...