Serious complications result from the battle that follows HIV infection because of the depressed functioning of the patient's immune system. This deterioration of the body's ability to defend itself from infectious disease is due in large part to the significant casualties of active T lymphocytes. In HIV patients, the virus causes uninfected T lymphocytes to self-destruct by a mechanism called apoptosis. Apoptosis, often called programmed cell death, is the term for the process by which normal cells kill themselves after receiving a death signal from other cells. Some cells, including cytotoxic T lymphocytes, contain a molecule known as Fas receptor.
Apoptosis is actually very important in the development and everyday function of an organism.
For instance, programmed cell death is the agent that removes the webbing between our fingers and toes during early development. Most cancerous cells are also given the signal to sacrifice themselves so that they do not damage the organism as a whole. The Fas mechanism of apoptosis is used in many physiological processes, including the control of T cell population. This is essential because T cells are constantly replicating and increasing their population. To maintain immune homeostasis, it is essential that this population of T cells be controlled, else these immunological warriors overrun the entire organism they are trying to defend.