The red blood cells used for screening patients' serum for unexpected antibodies should be of what blood group?

• A. A
• B. B
• C. O
• D. AB

Answer: (C)

Screening for unexpected antibodies uses red blood cells that won’t express A or B antigens, so any anti-A or anti-B antibodies in the patient’s serum won’t cause reactivity. Group O cells have no A or B antigens but still present a wide array of other clinically significant antigens (Rh, Kell, Duffy, Kidd, MNS, etc.), making them ideal for revealing antibodies against these non-ABO antigens. If cells expressing A or B were used, ABO antibodies in the serum could react with those antigens and complicate interpretation or create false-positive results, masking or confounding detection of antibodies to other antigens. Hence, the best choice is the cell group that lacks A and B antigens.