Which of the following antibodies is most often implicated in delayed hemolytic transfusion reactions?

The antibody most often implicated in delayed hemolytic transfusion reactions is Anti-E. Delayed hemolytic transfusion reactions typically occur days to weeks after a transfusion, primarily when the recipient has been previously sensitized to specific blood group antigens such as the Rh system. Anti-E is an antibody against the E antigen, which is relatively common and can be naturally occurring in individuals who lack the E antigen and may have been sensitized by prior transfusions or pregnancy.

When a patient who has anti-E receives a transfusion of E-positive blood, the antibody may not react immediately, as there is usually a low level of antibody present. However, after some time, especially if there is a secondary exposure, the patient's immune system can mount a response that leads to the destruction of the E-positive red blood cells, resulting in a delayed hemolytic reaction.

This is distinct from the other antibodies listed. For example, Anti-Jk3 and Anti-P1 are less commonly associated with transfusion reactions and do not generally lead to delayed hemolysis as frequently as Anti-E. Anti-e, while also a common antibody, has different characteristics regarding its immunogenicity and frequency of association with delayed reactions compared to Anti-E.