Answer:
Cell-mediated immunity is the function of phagocyte and cytotoxic T-cells. It does not involve antibodies and is specific for a particular antigen.
In HIV infection the virus enters into the T-helper cell(CD 4 cell) and reproduces in the T helper cell. They destroy the T helper cell during this process and then enter new CD 4 cell. By this process, they reduce the number of T helper cells.
T helper cell are important for cell-mediated immunity because it releases some chemicals that activate cytotoxic T cell and phagocytes which then kill the pathogens.
Therefore when the number of T helper cells reduces it becomes problematic for cell-mediated immunity to function properly.