This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
HLA-I is present on: 
Select and explain. 
A. All nucleated cells, B. Only on cells of immune system, C. Only on B cells, D. Only on T cells
A: All nucleated cells
Answer: A: All nucleated cells 
Explanation: Ans. is 'a' i.e., All nucleated cells(Ref: Jawetz, 27th/e, p. 139)* HLA class I Ag - present on the surfaces of all nucleated cells.* HLA class II Ag - only on cells of immune system (macrophages, dendritic cells, activated T cells and B cells).