T-Cells and the Immune System
The immune system protects the body from bacteria, viruses, toxins and other invaders. It also protects the body from cancer by attacking and destroying cancerous cells. Each cell in the body has a chemical marker that identifies that cell as belonging to that body. Anything the body sees as "other" is destroyed.
The immune system has several components that work together to protect the body---including a group of white blood cells called T-cells. There are three classes of T-cells: Helper T-cells, Killer T-cells and Suppressor T-cells.
In a healthy immune system the Helper T-cells act as guard dogs. When they detect an intruder they isolate the pathogen and send out a chemical alert. The Killer T-cells respond to the alert and destroys the pathogen. When the pathogen is destroyed, the Helper T-cells stop signaling and the Suppressor T-cells signal the Killer T-cells to stop killing. HIV specifically targets the Helper T-cells.
The HIV Virus
The HIV virus has a hard, protective shell with a single strand of RNA and a chemical called reverse transcriptase in its center. When HIV enters the blood stream and travels to lymph tissue (the lymph nodes and vessels, thymus and spleen) it binds with the Helper T-cells. The virus releases its RNA into the cell where reverse transcriptase converts the virus' RNA into DNA. This viral DNA rewrites the cell's genetic instructions and turns the cell into an HIV factory. As the cell reproduces, new copies of the HIV virus leave the cell, enter the lymph tissue and blood stream, and seek out healthy helper T-Cells.
In the first few weeks of infection, the virus releases millions of copies of itself into the blood. The remaining healthy Helper T-cells alert the immune system, which reacts by creating antibodies and sending Killer T-cells to fight the virus. During this time, the host gets sick with virus-like symptoms and the viral levels, in the blood, drop significantly as the immune system mounts its attack. However, while the immune system fights the virus in the blood, the virus continues to reproduce and attack healthy Helper T-cells in the lymph tissue. There are millions of Helper T-cells, but the virus is able to reproduce billions of copies of itself and overwhelm the Helper T-cells.
Over time the virus destroys the Helper T-cells so that they are no longer able to guard the system from infection.
Opportunistic Infections
Because the Helper T-cells are no longer able to do their job, infections that are normally kept in check by a healthy immune system take over. The other components of the immune system are still functional, they just don't perceive these infections as a threat, so the infections overwhelm the system.
The body is not only vulnerable to preexisting pathogens, it's also at risk from new infections. A new infection of the common cold is deadly because the immune system has no antibodies to the new strain and the Helper T-cells no longer alert to the new infection.
Cancers also run wild in an HIV-compromised system because the immune system no longer recognizes cancer cells as "other."
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