The immune system recognizes HIV by its antigen shape. But, just when the immune system recognizes one shape, HIV may multiply and change the shape of its antigen.
HIV can "hide" from the immune system for a long time.
When HIV infects a cell the virus may make more viruses and infect more cells or lie quietly in a cell for a long time. If HIV doesn't reproduce, the immune system can't find it and destroy it.
HIV destroys the immune system. The Immune system consists of Innate Immunity and Humoral Immunity Innate Immunity is Non-specific Immunity, whereas Humoral Immunity is specific Immunity. T-cells produce Cell-Mediated Immunity and B-cells produces Humoral Mediated Immunity.
As HIV destroys more and more helper T cells, the immune system begins to break down.
Finally the immune system can no longer kill HIV or any other invading germs and a person infected with HIV is diagnosed with AIDS.