HIV affects: March 2005 and 2008, and September 2011

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.