Continuing education episode! Usually, red cell alloantibodies are simple (find an antibody, give antigen-negative blood), but occasionally, some of them don't follow the rules! "Antibodies of variable clinical significance" act differently in different patients, sometimes causing hemolysis and sometimes not. They can be very hard to manage, especially because almost all blood donors are incompatible. The Monocyte Monolayer Assay, developed by today's guest, Sandy Nance, and her colleagues at the American Red Cross, helps answer the question, “Do we search the world for rare units of compatible blood, or can we just transfuse ABO-compatible red cells out of our regular inventory?” It's a great story, and this is a rare chance to hear about the MMA from one of the original sources, and to understand how to use it today.