Just to add to the conversation I'm in school to be a med tech and transfusion is one of the proficiencies! Its pretty cool how it works, basically after the donation we spin it down into packed red cells and plasma. Plasma gets pooled and treated and can be fractionated out into specific coagulation products or used as a suspension for platelets. Packed red cells are filtered (leukocyte reduced), sometimes treated or irradiated and have SAGM added (saline, adenine, glucose, mannitol) to stabilize the cells for storage.

In the lab we test and crossmatch to make sure everything is compatible before giving the ok! Sometimes it is just ABO and Rh grouping but there are a bunch of other antigens on red cells that recipients can develop antibodies to with repeat transfusion so sometimes we need to identify if that antibody is present and match with units that don't have it so it won't react.

There are also a bunch of complications with testing like nonspecific antibodies, cold reacting antibodies, subgroups of antigens, Its really interesting!