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
Marker for NK cell activity is- 
Select and explain. 
A. CD3, B. CD4, C. CD56, D. CDI9
C: CD56
Answer: C: CD56 
Explanation: T cell markers are unidigits (Eg: CD3) B cell markers are double digits (Eg: CD20) Natural Killer (NK) cell- 10-15% of peripheral blood lymphocyte Larger than small lymphocyte and contain azurophilic granules Large granular lymphocyte First line of defence against tumour cells, viral infected cells CD16 and CD56 positive Secretes gamma Interferon Antibody Dependent Cell mediated Cytotoxicity (ADCC)