ABO query

Jim79

if blood group O people have anti A and B antibodies in their plasma then why is O a 'universal' blood group?

Tree

Because O people can receive O cells. And A/B/AB don't have antibodies against O as it's the base structure from with A/B antigens are built.

O is the universal /donor/ rather than universal group. AB are universal recipients. ish.

Dingle_berry

The molecule that makes group O people group O is the precursor of the A and B molecules. So (almost) everyone has the O molecule and won't make antibodies to it. So it won't trigger a reaction in any recipient.

Group O people produce antibodies to the A and B molecules somewhere after 6 months of age. In most donors the level of these antibodies is low enough that after processing there isn't enough left to attack an A or B recipients cells. Occasionally the IBTS will receive a donation with high levels of anti A or anti B (or both). These units are clearly marked and only given to group O recipients.
If you followed all that then just to confuse you: the reverse situation applies to plasma. Group O are the universal recipient and group AB are the universal donors.