Antibodies in blood?

I had bloods done and I’ve been told I have antibodies in my blood, I’ve never been told this before and I used to be a donor and been pregnant before no issues. They’re saying it will affect what blood I’ll need if I need a transfusion? (I nearly did in my last birth as had a nearly 2litre pph which is why they’re checking now) I’ve googled it and can only find people who are rhesus or have had transfusions previously that have it, has anyone else had this and knows why?