In crossmatching, incompatibility in the antiglobulin phase is most commonly caused by which antibody?

• A. anti-K
• B. anti-D
• C. anti-Fya
• D. anti-Lu^a

Answer: (A)

When incompatibility is detected only in the antiglobulin (AHG) phase, it means an IgG antibody is binding to donor red cells and needs the AHG reagent to reveal the reaction. The Kell antigen system is highly immunogenic, and anti-K is a very common clinically significant IgG alloantibody. It binds to K antigen on donor cells and is most often detected during the AHG step, causing a crossmatch to appear incompatible unless a K-negative unit is used. Other listed antibodies can cause AHG-phase incompatibility as well, but anti-K is the most frequently encountered in this context. Lu^a antibodies tend to be IgM and react at cold temperatures (not AHG), and anti-D or anti-Fya, while capable of AHG reactions, are less commonly the cause of AHG-phase incompatibility in routine crossmatching compared with anti-K.