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
[{'content': 'Screening tests for B cell defects \nSelect and explain. \nA. Isohemagglutinin titers, B. CD 4 levels, C. Nitroblue tetrazolium dye test, D. Candida albicans intradermal skin test', 'role': 'user'}, {'content': 'Answer: A \nExplanation: T cell markers are unidigits (Eg: CD3) B cell markers are double digits (Eg: CD20)', 'role': 'assistant'}]