I'm about to go full nerd mode but anyways basically blood types are chosen by a chemical stamp so to speak on the red blood cells in your blood which effects who you can give blood to. The + or - is also called the RH factor which is named after a frog which basically means if you have it you can get it from + blood or - blood if you don't have it then you can only get it from - blood. With blood types O- is the universal donor since it can give to anyone with or without RH factor and it can give to type A, B, and AB. The universal recipient is AB + because that blood can get from any blood type and it has the RH factor so it can get from - or +. Thank you For your Time.
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