The human immunodeficiency virus (HIV), the pathogen that causes AIDS, both escapes and attacks the adaptive immune response. Once introduced into the body, HIV infects helper T cells with high efficiency by binding specifically to the CD4 accessory protein. HIV also infects some cell types that have low levels of CD4, such as macrophages and brain cells. Inside cells, the HIV RNA genome is reverse-transcribed, and the product DNA is integrated into the host cell’s genome. In this form, the viral genome can direct the production of new viruses. Although the body responds to HIV with an immune response sufficient to eliminate most viral infections, some HIV invariably escapes. One reason HIV persists is that it has a very high mutation rate. Altered proteins on the surface of some mutated viruses reduce interaction with antibodies and cytotoxic T cells. Such viruses replicate and mutate further. HIV thus evolves within the body. The continued presence of HIV is also helped by latency while the viral DNA is integrated in the host cell’s genome.
This latent DNA is shielded from the immune system as well as from antiviral agents currently used against HIV, which attack only actively replicating viruses.
Over time, an untreated HIV infection not only avoids
the adaptive immune response but also abolishes it. Viral replication and cell death triggered by
the virus lead to loss of helper T cells, impairing both humoral and cell-mediated immune responses. The eventual result is acquired immunodeficiency syndrome (AIDS), an impairment in immune responses that leaves the body susceptible to infections and cancers that a healthy immune system would usually defeat. For example, Pneumocystis carinii, a common fungus that does not cause disease in healthy individuals, can result in severe pneumonia in people with AIDS. Such opportunistic diseases, as well as nerve damage and body wasting, are the primary causes of death in AIDS patients, not HIV itself. Transmission of HIV requires the transfer of virus particles or infected cells from person to person via body fluids such as semen, blood, or breast milk. Unprotected sex (that is, without using a condom) and transmission via HIV contaminated needles (typically among intravenous drug users) cause the vast majority of HIV infections. The virus can enter the body through mucosal linings of the vagina, vulva, penis, or rectum during intercourse or via the mouth during oral sex. People infected with HIV can transmit the
disease in the first few weeks of infection, before they produce HIV-specific antibodies that can be detected in a blood test. Currently, 10-50% of all new HIV infections appear to be caused by recently infected individuals. Although HIV infection cannot be cured, drugs like (HAART) highly active antiretroviral therapy have been developed that can significantly slow HIV replication and the progression to AIDS.
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