Positive Kell antibodies are most likely due to exposure to which of the following?

• A. Exposure to previous blood transfusion
• B. Exposure to prior pregnancy
• C. Exposure due to autoimmune disease
• D. Exposure due to vaccination

Answer: (A)

Kell antibodies arise when someone who lacks the Kell antigen is exposed to Kell-positive red blood cells and mounts an immune response against that foreign antigen. The most common source of such exposure is a prior blood transfusion, where donor RBCs carrying the Kell antigen contact the recipient’s immune system and stimulate antibody formation. Exposure from a prior pregnancy can also occur, since fetal RBCs can enter maternal circulation and trigger alloimmunization, but this pathway is less frequent for Kell antibodies compared to transfusion. Autoimmune diseases produce autoantibodies against the person’s own cells, not alloantibodies to donor antigens, and vaccination is not a typical route to Kell antibody formation. Hence, previous transfusion is the most likely source.