This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'HIV affects: March 2005 and 2008, and September 2011 \nSelect and explain. \nA. CD4 cells, B. CD8 cells, C. Natural killer cells, D. Helper cells', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A: CD4 cells CD4 (cluster of differentiation 4) is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. CD4 is a primary receptor used by HIV-1 to gain entry into host T cells. HIV-1 attaches to CD4 with a protein in its viral envelope known as gp120. The binding to CD4 creates a shift in the conformation of gp120 allowing HIV-1 to bind to two other surface receptors on the host cell, the chemokine receptors CCR5 or CXCR4, depending on whether HIV is infecting a macrophage or T-helper cell. Following a structural change in another viral protein (gp41), HIV inses a fusion peptide into the host cell that allows the outer membrane of the virus to fuse with the cell membrane. HIV infection leads to a progressive reduction in the number of T cells possessing CD4 receptors. Therefore, medical professionals refer to the CD4 count to decide when to begin treatment for HIV-infected patients.', 'role': 'assistant'}]