Bacterial enzyme strips away blood types to create universal donor blood

Think of it like being allergic.

O types are allergic to everything. If they see an A or a B or both, they'll have a reaction.

A types are allergic to B. They have A riding along on their blood cells. Good times.

Similarly, B types are allergic to A.

AB abides all types.

So if you think of the reverse when donating blood, O types, since they're allergic to all types don't have any problems giving their blood since there's nothing for others to be allergic to. AB types are filthy and full of stuff others are allergic to, so they can only really share amongst themselves.

There's also +/- as an added complication.

This enzyme sounds pretty good as it'll just strip the stuff people could be allergic to. Put some AB blood in with the enzyme and you essentially get O blood anyone can use.

I secretly think your question was about a method of assassination. Receiving the antigens alone might not be a problem so much, but usually they're attached to red blood cells which your immune system will start fighting. Not good to have your body fight your blood.