Blood types!!

For simplicity purposes the fundamental blood types (without accounting for positive or negative) are:

A

B

AB

O

What distinguishes the different types of erythrocytes (red blood cells) is the "protein markers" or agglutinogens as they're specifically known.

So A type has A markers and so on.

However, O type has no markers at all.

This is important to your body because if you're type A then you cannot accept a donor with B or AB blood as your body will literally attack it (with your white blood cells) as it won't recognise or accept the blood and it'll deem it as an invader!

However, your body will accept O because O has NO protein markers, meaning your body can't distinguish the differentiation due to the fact that there are no markers to distinguish, so O is determined as being the "universal donor" because anybody can receive type O blood.

Type AB blood is therefore determined as being the "universal recipient" as it has both A and B protein markers so it will accept A, B, AB and O without any issue.

Here is a screen shot of an image from a YouTube video: