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The Immune System
When bacteria, viruses, or other agents invade the body, they are recognized as foreign, which then signals the immune system to attack and destroy them. If the immune system does not act sufficiently in response to an invading agent, the result will be infection. The interrelationships between cells in the immune system are complex and extremely intricate, and damage to one type of cell in the system compromises the entire system’s ability to protect the body from infections and cancers. How HIV affects the immune systemHIV is a retrovirus, a type of virus that stores its genetic information on a single-stranded RNA molecule. After a retrovirus penetrates a cell, it creates a DNA version of its genes, and its DNA becomes part of the infected cell’s DNA. HIV infects one particular type of immune system cell, called CD4 cells (or T-cells). T-cells coordinate immune regulation and secrete specialized factors that activate other white blood cells to fight off infection. In healthy individuals, the number of CD4 cells normally ranges from 450 to 1,200 cells per microliter of blood (this measurement is known as the T-cell count). When infected with HIV, a T-cell becomes an HIV-replicating cell. In other words, the virus binds with the cell, copies itself into the cell’s DNA, and causes the cell to begin producing new HIV viruses. This process eventually causes the cell to die. As the number of T-cells decreases, the infected person’s immune system becomes increasingly compromised. When a person’s T-cell count (number of CD4 cells) drops to below 200 cells per microliter of blood, the person is considered to have AIDS.
An infected person’s body tries to fight off HIV infection by aggressively manufacturing antibodies, which are tiny bits of protein designed to bond with HIV particles and neutralize them before they can infect more cells. (Most HIV tests actually detect the presence of these antibodies, not HIV itself.) It is particularly difficult for the immune system to fight off HIV infection for a number of reasons, including the following:
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The
immune system is composed of many interdependent cells and organs that
protect the body from bacteria, parasites, fungi, viruses, and tumor cells.
The immune system acts like an army that protects the body from invaders,
and each of the different cells of the immune system performs a highly
specialized and interrelated function in fighting off these invaders.
