I had a transfusion yesterday and they had to count my "antibodies" first...what exactly does that mean?

This was my 6th transfusion in less than 6 months (very anemic due to hepc treatment). They always say that with my type/cross they have to also count my "antibodies" in my blood. They just keep telling me it's something in my blood and that every time I get a transfusion, I probably get more antibodies. They say that's why it takes nearly 3 hours to type/cross me (which is bull and it sucks!). Does anyone know (in layman's terms please) what this means? Thanks in advance!