Final answer:
The delayed hemolytic transfusion reaction in a patient with an unknown blood type who received an O Rh+ blood transfusion is most likely due to the presence of pre-formed anti-Rh factor antibodies in the patient's serum.
Step-by-step explanation:
The component of the patient's serum most likely responsible for the delayed hemolytic transfusion reaction is anti-Rh factor antibodies. When a person with Rh- blood is exposed to Rh+ blood, like in the case of the emergency transfusion with type O Rh+ blood, the individual's immune system may form these antibodies. This is because the Rh antigen, especially the Rh D or Rho/D antigen, is highly immunogenic.
In the scenario described, the delayed hemolytic transfusion reaction suggests that the patient was likely previously sensitized to the Rh antigen and their immune system had generated IgG anti-Rh antibodies. When transfused with Rh+ blood, these pre-formed antibodies would not cause an immediate reaction but could lead to a delayed hemolytic response within a few days to weeks as the antibodies bind to the Rh+ donor red blood cells and lead to their destruction.