This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cells and Tissues of Immune System, Subject name: Pathology
[{'content': 'CD4 is a marker for? \nSelect and explain. \nA. Stem cells, B. All leukocytes, C. B - cells, D. T - cells', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is 'd' i.e., T - cells o See above explanation.", 'role': 'assistant'}]