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Can you explain AIDS and how it affects the immune system? How does HIV become AIDS?

William Chris Woodward, Medical Director at Bornemann Internal Medicine in Reading, Pa., relays the following description:

HIV infection mechanism
Image: SCIENTIFIC AMERICAN
HIV INFECTION begins with a sharp rise of virus in the blood (orange line) and a consequent drop in CD4 T cells (blue line). The immune system soon recovers somewhat, however, and keeps HIV levels fairly steady for several years. Eventually, though, the virus gains the upper hand. AIDS is diagnosed when the CD4 T cell level drops below 200 cells per cubic millimeter of blood or when opportunistic infections arise.

The acquired immune deficiency syndrome (AIDS) was first recognized in the early 1980s. AIDS is caused by the human immunodeficiency virus (HIV) and is spread through the exchange of body fluids (sexual encounters, sharing needles, blood transfusions). Recent research suggests the virus "jumped" to humans from a West African subspecies of chimpanzee (Pan troglodytes troglodytes) intermittently decades or even centuries ago. The World Health Organization (WHO) estimates that millions are infected with HIV worldwide and that it is the most devastating epidemic since the influenza pandemic of 1918. There are some predictions that HIV will not be controlled until the middle of the next century and that it may continue to devastate developing countries for the next 100 years.

HIV is a unique human RNA virus, capable of infecting cells of the immune system. Specifically, HIV targets T helper cells (CD4 cells), leading to the eventual death of the cell. CD4 cells are vital players in the regulation of immune responses to invading microorganisms. In an untreated person, 10 billion to 100 billion new viruses are produced per day. This massive viral replication leads to a progressive loss of CD4 cells over a period of several years to as long as a decade. And destruction of CD4 cells renders a patient vulnerable to unusual opportunistic infections (OIs) that are rarely seen in healthy humans. Most patients who die from AIDS succumb to one or more OIs.

drug chart

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DRUG RESISTANCE is a major problem in HIV infections. If patients do not adhere perfectly to their drug regimen, the virus rapidly eliminates its vulnerability. Once it has evaded one drug combination, others are less likely to work as well.

AIDS denotes the later stages of the disease and is not diagnosed until the patient has developed a significant OI or the CD4 cell count in the bloodstream falls below 200 (normal is 500 to 1,000 cells per milliliter). Therefore, infection with HIV does not necessarily mean AIDS, but all patients with AIDS have HIV infection.

Until 1996, HIV infection was fatal in the vast majority of infected individuals. With recent advances in understanding the virus life cycle, how and where the virus damages the immune system, and the action of new drugs, however, the course of AIDS in many patients in the developed world has dramatically changed. In fact, patients are doing so well that the term AIDS may have outlived its usefulness.

Combinations of drugs such as nucleoside reverse transcriptase inhibitors and protease inhibitors can help control viral replication, restore immune function and maintain health. We have seen patients literally on their deathbed return to full-time employment. The bad news is that long-term toxicity to virtually all these drugs has increasingly been recognized as patients take these medications for longer periods of time. In addition, patients must take the combination (commonly called HAART, for highly active anti-retroviral therapy) exactly as prescribed. If adherence to the regimen is not perfect, HIV can quickly become resistant to the medication. And once an initial combination fails, it is less likely a second, different combination will be effective. The good news is that newer drugs active against resistant viral strains and newer approaches to treatment are on the way. It is also important to note that HIV research may lead to advances in the treatment of other viral infections, as well as cancers, metabolic diseases (diabetes, high cholesterol) and other immune system disorders.

The major needs for the future include an effective preventive vaccine, new drugs, better understanding of the long-term side effects of the current drugs and improved health care delivery to people in the developing world. Perhaps the most important short-term need is effective prevention strategies. AIDS is a preventable infection; better prevention will significantly decrease the tremendous burden HIV infection places on humans around the globe.

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