How does HIV reduce immune function?

HIV infects and kills helper T-cells. These cells are central to a proper adaptive immune response. Helper T cells are the component of adaptive immunity that is stimulated by antigen presenting cells during an infection. Once activated, helper T-cells can activate cytotoxic T-cells to initiate a cell mediated response and B cells to initiate a humoral response. Over time, an HIV infection can result in a greatly reduced number of helper T cells. Consequently, although healthy populations of cytotoxic T-cells and B-cells remain, without enough helper T-cells they cannot be activated properly, and both cell mediated and humoral responses decline, opening the door to the opportunistic infections that are characteristic of advanced AIDS.