The Human Immunodeficiency Virus belongs to the lentivirus subfamily of retroviruses. Structurally, the virion is spherical and encased in a lipid bilayer stolen from the host cell membrane. Embedded within this envelope are viral glycoproteins, specifically gp120 and gp41, which are critical for recognizing and attaching to target human cells. Beneath the viral envelope lies a protein matrix, and inside that core sits the viral capsid made of p24 protein. This core protects the most vital cargo: two identical strands of single-stranded positive-sense RNA and the enzymes required for replication, namely reverse transcriptase, integrase, and protease. Unlike typical DNA viruses, retroviruses use reverse transcriptase to convert their genetic RNA material into DNA once inside a host cell.
Human Immunodeficiency Virus transmission requires contact with specific infected body fluids, primarily blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. The primary modes of acquisition include unprotected sexual intercourse, vertical transmission from mother to child during pregnancy, birth or breastfeeding, and parenteral exposure through contaminated needles or blood products.
Cellular tropism refers to the specific cell types the virus infects. HIV displays a strong preference for cells expressing the CD4 surface receptor, predominantly CD4+ T-lymphocytes, macrophages, and dendritic cells. Infection begins when the viral surface protein gp120 binds to the host CD4 receptor. This binding induces a conformational change that allows gp120 to interact with a secondary coreceptor, usually CCR5 or CXCR4, facilitating viral entry into the host cell cytoplasm.
Once inside the host cell, the viral RNA is reverse-transcribed into double-stranded DNA. This newly formed viral DNA is transported into the nucleus, where the integrase enzyme inserts it into the host's cellular genome, becoming a provirus. The host machinery then transcribes the provirus, producing new viral RNA and proteins. The protease enzyme cleaves these long protein chains into functional components, allowing new virions to assemble and bud out from the cell membrane.
Continuous viral replication leads to the progressive loss of CD4+ T-cells through several mechanisms:
The natural history of an untreated HIV infection progresses through three distinct clinical phases:
The World Health Organization utilizes a clinical staging system to guide patient management, especially in resource-limited settings where routine CD4 testing may not always be immediately accessible. This system categorizes disease severity into four distinct stages based on clinical manifestations:
Immunological decline in HIV infection is a steady, cumulative process. As CD4+ T-cell numbers dwindle, the architecture of secondary lymphoid organs becomes progressively damaged, leading to generalized immune dysfunction. Beyond sheer numerical loss, the remaining T-cells exhibit impaired functional capacity, failing to proliferate or produce essential cytokines when challenged by pathogens.
In addition to T-cell depletion, B-lymphocyte function is disrupted, resulting in hypergammaglobulinemia alongside an inability to mount effective new antibody responses against novel vaccines or infections. When the immune system can no longer maintain surveillance, the body loses control over latent viruses and environmental microbes. This profound collapse marks the clinical threshold of AIDS, leaving the patient vulnerable to life-threatening complications that rarely affect individuals with intact immunity.
When cell-mediated immunity fails, patients develop signature illnesses known as opportunistic infections and malignancies. Common microbial invaders include Pneumocystis jirovecii, which causes a severe interstitial pneumonia, and Cryptococcus neoformans, responsible for fungal meningitis. Parasitic threats like *Toxoplasma gondii* frequently cause focal neurological lesions, while reactivation of latent Mycobacterium tuberculosis leads to widespread pulmonary and extrapulmonary disease.
Oncogenic viruses also exploit the lack of immune surveillance. Persistent infection with human herpesvirus 8 drives the development of Kaposi's sarcoma, a vascular tumor presenting as purplish skin lesions and visceral involvement. Additionally, immunosuppressed patients face a markedly elevated risk for aggressive blood cancers like non-Hodgkin lymphoma, highlighting the immune system's critical role in suppressing malignant cell growth.