HIV
Retrovirus causing AIDS, transmitted via bodily fluids.
Human immunodeficiency viruses (HIVs) are two species of Lentivirus, a subgroup of retrovirus, that infect humans and cause acquired immunodeficiency syndrome (AIDS). Over time, HIV leads to progressive failure of the immune system, allowing life-threatening opportunistic infections and cancers to thrive. Without treatment, average survival time after infection is estimated to be 9 to 11 years, depending on the HIV subtype.
HIV is transmitted primarily through sexual contact, as well as via blood, semen, pre-ejaculate, and vaginal fluids. Non-sexual transmission can occur from an infected mother to her child during pregnancy, childbirth, or breastfeeding. The virus exists in these fluids both as free particles and within infected immune cells. Notably, research has demonstrated that HIV is not contagious during sexual intercourse without a condom when the infected partner maintains a consistently undetectable viral load, a finding that applies to both same-sex and opposite-sex couples.
The virus targets key immune cells, including CD4+ helper T cells, macrophages, and dendritic cells. Infection depletes CD4+ T cells through several mechanisms: pyroptosis of abortively infected cells, apoptosis of uninfected bystander cells, direct viral killing, and destruction by CD8+ cytotoxic lymphocytes. When CD4+ counts fall below a critical threshold, cell-mediated immunity collapses, leaving the body vulnerable to opportunistic infections and cancers, culminating in AIDS.
Virologically, HIV is a lentivirus, a genus of retroviruses known for causing long-term illnesses with extended incubation periods. Its genome consists of two copies of single-stranded, positive-sense RNA, enclosed within a capsid made of p24 protein, surrounded by a p17 matrix, and enveloped by a lipid bilayer derived from the host cell. The envelope contains glycoproteins gp120 and gp41, which facilitate attachment and fusion with target cells. Upon entry, reverse transcriptase converts viral RNA into double-stranded DNA, which integrase then inserts into the host genome. The virus can remain latent for up to ten years without symptoms, or it can actively replicate, producing new virions.
Two types exist: HIV-1, more virulent and globally dominant, and HIV-2, less infectious and largely confined to West Africa. In 2020, the International Committee on Taxonomy of Viruses reclassified them as *Le
- Classification
- Genus Lentivirus, family Retroviridae
- Types
- HIV-1 and HIV-2
- Transmission
- Sexual contact, blood, pre-ejaculate, semen, vaginal fluids; mother-to-child during pregnancy, childbirth, or breastfeeding
- Target cells
- Helper T cells (CD4+ T cells), macrophages, dendritic cells
- Genome
- Single-stranded positive-sense RNA, nine genes
Lore & Background
HIV is a member of the genus Lentivirus, part of the family Retroviridae. Lentiviruses are transmitted as single-stranded, positive-sense, enveloped RNA viruses. Upon entry into a target cell, the viral RNA genome is reverse transcribed into double-stranded DNA by the enzyme reverse transcriptase, then integrated into the cellular DNA by integrase. The virus may become latent, avoiding immune detection for up to ten years after primary infection, during which it causes no symptoms. Alternatively, integrated viral DNA can be transcribed to produce new virus particles. Two types of HIV have been characterized: HIV-1 and HIV-2. HIV-1 is more virulent and infective, causing the majority of global infections. HIV-2 is largely confined to West Africa due to its relatively poor capacity for transmission. The viral envelope, derived from the host cell membrane, contains envelope proteins (gp120 and gp41) that allow attachment and fusion with target cells. The envelope protein is a major target for vaccine efforts due to its dense glycosylation, which shields the virus from antibody neutralization.
Reader's Guide
HIV is a significant pathogen because it causes AIDS, a condition that progressively destroys the immune system, leading to susceptibility to opportunistic infections and cancers. Without treatment, average survival time after infection is 9 to 11 years. HIV is primarily a sexually transmitted infection but can also be transmitted via blood, from mother to child during pregnancy, childbirth, or breastfeeding. Research has shown that HIV is not contagious during sexual intercourse without a condom if the HIV-positive partner has a consistently undetectable viral load. The virus infects vital immune cells, including CD4+ T cells, macrophages, and dendritic cells. It reduces CD4+ T cell levels through mechanisms such as pyroptosis, apoptosis, direct viral killing, and killing by cytotoxic lymphocytes. When CD4+ T cell numbers decline below a critical level, cell-mediated immunity is lost, leading to AIDS. HIV's ability to integrate into host DNA and remain latent complicates treatment and eradication. The envelope protein's dense glycosylation poses challenges for vaccine development, though recombinant trimeric viral spikes (SOSIP trimers) are promising vaccine candidates. The virus's genetic complexity, with nine genes encoding 19 proteins, allows it to regulate replication, evade immune responses, and cause disease.
Did You Know?
- HIV is present in bodily fluids as both free virus particles and virus within infected immune cells.
- HIV-1 is more virulent and infective than HIV-2, which is largely confined to West Africa.
- The viral envelope protein is one of the most densely glycosylated molecules known, with glycans shielding it from antibody neutralization.
- The virus can remain dormant in the human body for up to ten years after primary infection without causing symptoms.
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