system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when your injected with blood of a incompatible type?
gpt
You must first understand how bloodworks:
Type a blood has an "A" antigen(basically a license plate for your cells to tell your immune system what they do)
Type B blood only has a "B" antigen
Type AB has both an "A" and a "B" antigen
Type O has none of these antigens

So your immune system is good at attacking anything that it doesn't see as normal. It is afraid of change so it will kill bacteria and stuff but also any foreign cells. So your immune system uses the antigens to see if the blood is the same type as it. If you have type O(no antigens) but your immune system sees a type A blood cell then it will pour all its resources into getting rid of the type A blood cells and you will get sick from your immune system not being able to do more important things. 

Now let's talk about donations: if you have type O blood then you can donate your blood to anybody(no matter the blood type of the recipient ) and type O people are known as universal donors for this. They can donate to anybody because their blood has no antigens and thus can't be recognized as foreign to the immune system(it's a bit like taking off your license plate because the police can't identify you). If you have type AB blood you are a universal recipient because you have both A and B antigens so your body won't attack the blood because it thinks it's natural. 

I hope that helped
Source:I am in a college Hematology class.