HIV and your immune system
HIV is dangerous because the virus destroys the very cells of your immune system that are supposed to be keeping you well: the CD4 cells.
HIV attaches itself to a CD4 cell and enters. It makes copies of itself inside the CD4 cell and then goes on to damage and destroy the cell. The new HIV viruses burst out of the CD4 cell and go off to find more cells to invade.
If the number of CD4 cells is reduced, the immune system has fewer cells to help it defend the body from other organisms. This means you are at greater risk of getting ill.
The immune system fights HIV infection by producing antibodies, but they are not effective without the CD4 cells.