What Is Human Immunodeficiency Virus?

Human immunodeficiency virus (HIV) is a retrovirus that infects cells of the human immune system, primarily CD4+ T lymphocytes, and progressively depletes them, leading to the condition known as acquired immunodeficiency syndrome (AIDS) when the immune system can no longer mount effective defenses against opportunistic infections. HIV belongs to the genus Lentivirus within the family Retroviridae and exists in two main types: HIV-1, which accounts for the global pandemic, and HIV-2, which is less transmissible and largely confined to West Africa. The virus is transmitted through specific bodily fluids including blood, semen, vaginal secretions, rectal fluids, and breast milk.

HIV sits at the intersection of virology, immunology, and biomedical engineering. Since the epidemic's recognition in the early 1980s, research in molecular biology has mapped the virus's replication cycle in detail, enabling the rational design of antiretroviral drugs that interrupt it at multiple stages. Biomedical engineering contributes diagnostic platforms, drug delivery systems, and the three-dimensional tissue models that support pre-clinical research.

Virology and Pathogenesis

HIV is an enveloped RNA virus with a genome of approximately 9.7 kilobases encoding nine genes. Its outer envelope carries glycoprotein spikes (gp120/gp41) that bind to the CD4 receptor and either the CCR5 or CXCR4 co-receptor on target cells. After entry, the viral enzyme reverse transcriptase converts the RNA genome into double-stranded DNA, which is then integrated into the host cell's chromosomal DNA by the viral integrase enzyme. The NIH overview of the HIV life cycle describes each of the seven stages, from binding through budding, that are targeted by approved therapeutic agents.

Without treatment, HIV infection progresses through three phases: acute infection characterized by rapid viral replication and a transient drop in CD4 count; a chronic phase lasting years during which the virus continues replicating at lower levels; and the symptomatic phase in which CD4 counts fall below 200 cells per microliter and AIDS-defining illnesses emerge.

Antiretroviral Therapy

Antiretroviral therapy (ART) uses combinations of drugs from at least two mechanistic classes to suppress viral replication below detectable levels. The six principal drug classes approved by the FDA target different stages of the HIV life cycle: nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) block reverse transcription; integrase strand transfer inhibitors (INSTIs) prevent chromosomal integration; protease inhibitors disrupt the maturation of new viral particles; entry inhibitors block receptor binding or membrane fusion; and CCR5 antagonists prevent co-receptor engagement.

The PMC review of HIV-1 antiretroviral drug therapy documents how combination ART, first introduced in 1996 as highly active antiretroviral therapy (HAART), reduced AIDS-related mortality in treated populations by more than 90 percent. Modern first-line regimens often consist of a single daily co-formulated tablet combining two NRTIs and an INSTI.

Vaccine and Cure Research

Despite decades of research, no preventive HIV vaccine has reached licensure. The difficulty stems from the virus's high mutation rate, its ability to establish latent reservoirs in resting CD4 cells that persist through ART, and the diversity of HIV strains circulating globally. The NCBI reference on HIV-1 describes the viral genetic variability that complicates both vaccine antigen design and long-term therapeutic strategies.

Functional cure research targets the latent reservoir through "shock and kill" strategies that activate latent provirus to allow immune clearance, and "block and lock" approaches that enforce permanent silencing of the provirus. A small number of patients who received allogeneic bone marrow transplants from donors homozygous for the CCR5-delta32 deletion have achieved apparent sterilizing cures, providing proof of concept for CCR5-targeted interventions.

Applications

Human immunodeficiency virus research has applications in a wide range of fields, including:

• Point-of-care diagnostics for viral load and CD4 count monitoring in resource-limited settings
• Drug delivery system design for sustained-release antiretroviral formulations
• Biomedical engineering models of HIV transmission and immune dynamics
• mRNA vaccine platform development, informed by HIV immunogen engineering
• Prevention programs combining pre-exposure prophylaxis (PrEP) with behavioral interventions