The immune systemthe body's defense against disease and illnessis a complex network of organs, cells, and proteins that
- defends the body against invasion by foreign disease-bearing organisms, such as HIV;
- identifies and destroys abnormal cells, such as cancer cells; and
- flushes dead and damaged cells out of the body.
A healthy immune system readily recognizes and identifies disease-causing bacteria, viruses, fungi, and other abnormal cells. When an abnormality is detected, messages are sent from cell to cell to invoke an "attack" to destroy the abnormal cells. Afterwards, the immune system "remembers" the ordeal so that, if the same pathogen (disease-causing organism) invades the body again, it can quickly and readily come to the body's defense. This remarkable ability to remember a foreign substance and how to attack it is the basis of vaccination.
The immune system is made up of a variety of different types of cells (e.g., leuckocytes, lymphocytes, phagocytes, B cells, CD8+ cells). HIV targets CD4+ cells, also called helper T-cells. Health care professionals often use the terms "CD4+ cell" and "T-cell" interchangeably. Helper T-cells are immune system managers. They "instruct" other cells when to start and stop carrying out their various duties.
HIV interferes with the immune response in the following ways:
- HIV replicates more quickly than the immune system cells and challenges the immune system's speed and efficacy.
- HIV targets CD4+ cells, the cellular managers of the immune system, destroying their ability to activate other immune system cells.
- HIV destroys the CD4+ cells and puts demands on the body to replace them.
The immune system can replace as many as 10 billion CD4+ cells a day; but, after years of fending off the virus, it begins to wear down. The gap between the number of cells destroyed and the number that can be replaced grows wider over time and eventually leads to AIDS.
Healthy, uninfected people have between 800 and 1200 CD4+ cells per mm of blood. HIV causes the number of CD4+ cells to decline to dangerously low levels, making a person infected with the virus very vulnerable to the opportunistic infections (e.g., cancers, neurological conditions, diarrhea, weight loss) that characterize AIDS. This also puts the person at high risk for unusual infections. According to the Centers for Disease Control, a CD4+ cell count below 200/mm is a criterion for AIDS.