Which antibody is usually IgG, immune, and antiglobulin phase reactive?

• A. anti-Fy^a
• B. anti-Jk^a
• C. anti-D
• D. anti-K

Answer: (B)

Antibodies that are IgG, formed after exposure to an antigen (immune), and detected only by the antiglobulin test are the ones most often implicated in clinically significant transfusion reactions. Anti-Jk^a fits this pattern best: it is predominantly an IgG alloantibody formed after exposure to the JK antigen, and it typically shows reactivity at the antiglobulin (AHG) phase. This AHG-detectable, IgG, immune nature makes it a classic example of an antibody that is clinically significant and requires the indirect Coombs test for demonstration. While other antibodies listed can also be IgG and AHG reactive, anti-Jk^a is the textbook instance that exemplifies this combination most clearly.