This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunity, Subject name: Pathology
Which is a flow cytometric B cell marker? 
Select and explain. 
A. CD 2, B. CD 3, C. CD 7, D. CD 19
D: CD 19
Answer: D: CD 19 
Explanation: * B cell markers are CD 19, 20, 21 and 23.* TH cell markers are CD 4, 3, 7, 2* TC cell markers are CD 8, 3, 7, 2* NK cell markers are CD 2, 16, 56