How HIV/AIDS Works

HIV targets a specific type of white blood cell called CD4+ T cells, which act as generals coordinating immune responses throughout the body. On the surface of the virus is a protein called gp120 that has evolved to fit perfectly into CD4 receptors on these T cells, much like a key sliding into a lock. This initial binding is highly specific—HIV cannot infect cells without CD4 receptors, which is why the virus has such a narrow target.

The attachment process doesn't stop with CD4 alone. After the initial binding, the virus must also attach to a co-receptor, typically either CCR5 or CXCR4, which are also present on the surface of CD4+ T cells. This two-step authentication system ensures the virus is docked securely to the cell membrane. Some people have genetic mutations that alter their CCR5 co-receptors, making them naturally resistant to most HIV strains—a fascinating example of human genetic variation affecting disease susceptibility.

Once firmly attached through both receptors, the viral envelope begins to fuse with the cell membrane. This fusion process brings the outer coating of the virus into direct contact with the cell's outer wall, creating an entry point. The entire binding mechanism is so efficient that HIV can attach and prepare for entry within minutes of encountering a suitable target cell.

After fusion with the cell membrane, HIV releases its contents into the cell's cytoplasm—the fluid interior where cellular machinery operates. The viral core, a cone-shaped protein shell called the capsid, enters the cell carrying two strands of HIV's genetic material in the form of RNA. Unlike human cells that store genetic information as DNA, HIV initially carries its instructions as the related molecule RNA, requiring an extra conversion step to hijack human cellular systems.

Along with the RNA, the virus injects several specialized enzymes it will need to complete its takeover. The most critical of these is reverse transcriptase, an enzyme that can perform a trick human cells cannot: converting RNA back into DNA. The virus also includes integrase, which will later splice viral DNA into the host genome, and protease, which will help assemble mature virus particles. These enzymes are pre-packaged with the viral RNA because the host cell doesn't naturally produce them.

This injection phase is remarkably vulnerable, and it's one reason why the virus includes multiple RNA copies—if one strand is damaged, the other serves as backup. The viral contents are now floating in hostile territory, surrounded by cellular defenses that can detect and destroy foreign genetic material. The race is on for HIV to convert its RNA into DNA before the cell's defenses neutralize it.

Once inside the cell, reverse transcriptase immediately begins transcribing HIV's RNA into DNA, working like a biological photocopier that translates from one format to another. This process is notoriously error-prone—reverse transcriptase makes about one mistake per 10,000 nucleotides copied, which means nearly every new DNA copy contains at least one mutation. While this sloppiness might seem like a weakness, it's actually HIV's superpower: these constant mutations allow the virus to rapidly evolve and evade both immune responses and antiviral drugs.

The newly synthesized viral DNA must now reach the cell's command center—the nucleus, where the cell's chromosomes reside. The viral DNA travels to the nucleus still associated with viral proteins in what's called the pre-integration complex. In most cells, the nuclear envelope poses a formidable barrier, but HIV can pass through nuclear pores or, in dividing cells, access chromosomes when the nuclear envelope temporarily breaks down during cell division.

Inside the nucleus, the integrase enzyme cuts open the cell's chromosomal DNA and inserts the viral genetic code directly into the host genome. This integration is permanent—once HIV's DNA becomes part of a chromosome, it will be copied every time that cell divides, and it cannot be removed by any known medical treatment. The cell has been transformed into a viral factory, with instructions to produce HIV now written into its genetic blueprint alongside genes for normal cellular functions.

Once integrated into the host chromosome, HIV's DNA sits dormant until the cell becomes activated—typically when the immune system calls CD4+ T cells into action to fight an infection. When the cell activates, it reads HIV's genes just as it would its own, producing long strings of viral RNA and proteins. The cell's own ribosomes, which normally manufacture proteins for cellular functions, are now unwittingly building HIV components: envelope proteins, capsid proteins, and the enzymes needed for new viral particles.

The viral components initially emerge as long, non-functional protein chains that must be cut into working pieces. This is where the protease enzyme becomes critical—it acts as molecular scissors, cleaving the protein chains at specific points to create functional viral proteins. Without protease activity, the viral proteins remain in useless, tangled strings, which is why protease inhibitors are such effective anti-HIV medications.

New viral particles begin assembling just beneath the cell's membrane, where viral RNA, enzymes, and structural proteins come together in a coordinated construction process. A single infected CD4+ T cell can produce thousands of new viral particles over its lifetime. These immature virions bud off from the cell membrane, wrapping themselves in a piece of the host cell's outer layer studded with HIV envelope proteins. As they break free, the protease enzyme performs final maturation cuts, transforming immature particles into fully infectious viruses ready to attack new CD4+ cells.

CD4+ T cells don't just get infected—they die from the process. The virus kills these cells through multiple mechanisms: direct viral destruction when particles burst from the cell, toxic effects from viral proteins accumulating inside, and triggering of cellular suicide pathways called apoptosis. In an uninfected person, CD4+ T cell counts range from 500 to 1,500 cells per microliter of blood, but HIV steadily drives these numbers down. When counts drop below 200 cells per microliter, the immune system has been so compromised that a person is diagnosed with AIDS.

The impact of losing CD4+ T cells is catastrophic because these cells serve as the immune system's coordinators. They activate other immune cells, orchestrate responses to pathogens, and maintain immunological memory of past infections. As their numbers dwindle, the entire immune defense network collapses. Infections that healthy immune systems easily control—like the fungus that causes thrush or the parasite that causes toxoplasmosis—become life-threatening. Certain cancers, especially those linked to other viruses, also become more common because immune surveillance against abnormal cells fails.

Without treatment, HIV typically depletes CD4+ cells over a period of years, progressing through stages of gradually worsening immune function. Modern antiretroviral therapy works by blocking various steps in HIV's replication cycle, preventing new rounds of infection and allowing CD4+ cell populations to recover. When viral replication is suppressed, people with HIV can maintain healthy immune function and normal lifespans, though they remain infected because the virus persists in its integrated form within long-lived cells.