In which scenario is anti-D most likely to develop?

Anti-D is an antibody that specifically targets the D antigen found on the surface of red blood cells, which is a key component in determining a person's Rh factor. The development of anti-D antibodies is most likely to occur when an Rh-negative individual is exposed to Rh-positive blood.

When Rh-positive blood is transfused into an Rh-negative individual, the recipient’s immune system recognizes the D antigen as foreign and can mount an immune response, leading to the production of anti-D antibodies. This is particularly significant in cases of blood transfusion or in pregnancy, where an Rh-negative mother could develop antibodies against Rh-positive fetal cells.

The other scenarios do not lead to the development of anti-D antibodies. For instance, when Rh-negative blood is given to an Rh-negative individual, there is no foreign D antigen present; thus, no immune response is triggered. Similarly, if Rh-positive blood is given to another Rh-positive individual, there is no foreign mechanism to activate the immune system. Lastly, giving Rh-negative blood to an Rh-positive person also does not trigger a response since the recipient already has the D antigen and will not identify it as a foreign threat.

Therefore, the scenario where anti-D is most likely to develop is when Rh-positive blood is administered