Although B-cells produce antibodies, for some reason they are not totally effective. The entire immune system is compromised. When that happens, a virus or other disease-causing organism that would normally be stopped can become deadly. For instance, almost all adults are infected with herpes simplex virus, but typically it causes no more than a cold sore. In a person with HIV, herpes simplex can overwhelm the system, causing a severe, even fatal, infection.
Several theories propose that the immune system turns on itself.
These antibodies then attack healthy immune system cells and other cells that contain the same surface proteins.