There are three components that can cause problems with transfusions: "A", "B" and "Rhesus". A person can have any combination of those three (8 combinations).
The A/B/AB/O is basically having only A, only B, both, or neither. And the +/- is whether you have the "Rhesus" component.
The basic rule is that you can't introduce new components to a person who doesn't have them in their system, and will be treated as foreign contaminants.
So someone with all three components already (AB+) can take any blood because their system already is used to all three components.
A person with none of the components in their system (O-) would have an allergic reaction to any of the three components being introduced. On the other hand their blood is safe to donate to anyone else as it won't introduce any "unexpected components" to a recipient.
EDIT: ah, I see - article doesn't mention Rhesus. I guess it can convert AB- to O-, or AB+ to O+.
The A/B/AB/O is basically having only A, only B, both, or neither. And the +/- is whether you have the "Rhesus" component.
The basic rule is that you can't introduce new components to a person who doesn't have them in their system, and will be treated as foreign contaminants.
So someone with all three components already (AB+) can take any blood because their system already is used to all three components.
A person with none of the components in their system (O-) would have an allergic reaction to any of the three components being introduced. On the other hand their blood is safe to donate to anyone else as it won't introduce any "unexpected components" to a recipient.
EDIT: ah, I see - article doesn't mention Rhesus. I guess it can convert AB- to O-, or AB+ to O+.