This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immune system, Subject name: Medicine
Screening tests for B cell defects 
Select and explain. 
A. Isohemagglutinin titers, B. CD 4 levels, C. Nitroblue tetrazolium dye test, D. Candida albicans intradermal skin test
A: Isohemagglutinin titers
Answer: A: Isohemagglutinin titers 
Explanation: T cell markers are unidigits (Eg: CD3) B cell markers are double digits (Eg: CD20)