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Comprehensive and Detailed Explanation:

A positive antibody screen with reactivity against all reagent screening cells but a negative autocontrol suggests the presence of an alloantibody directed against a high-frequency antigen. High-frequency antigens are present on nearly all normal red blood cells, so antibodies against these antigens may react with most screening and panel cells.

The negative autocontrol indicates that the patient’s own red blood cells are not coated with the antibody, making an autoantibody less likely. Additional investigation may require testing with rare reagent cells, reviewing patient transfusion history, and performing specialized antibody identification procedures.

Examples of antibodies against high-frequency antigens include antibodies associated with the Vel, Kpb, and Lub blood group systems.

MLS professionals must recognize these patterns because failure to identify these antibodies can result in delays obtaining compatible blood and may lead to transfusion reactions.