There are many different sub-categories of blood groups. The main ones everyone knows are A, B, AB, and O plus the rhesus positive or negative. The anitbody screen looks for antibodies produced when you have been exposed to a different blood group either through pregnancy or blood transfusion for example. As these antibodies to minor blood groups can cause problems during or after pregnancy, eg severe jaundice in the baby. An example of one is anti-kell.