Human immunodeficiency virus (HIV) is a retrovirus that attacks the body’s immune system, making it difficult to fight off other illnesses. It operates by inserting its genetic instructions into the host’s DNA, fundamentally altering the body’s defensive capabilities. If left untreated, this progressive destruction can lead to acquired immunodeficiency syndrome (AIDS), a severe stage where the immune system is significantly compromised.
The Primary Target: CD4+ T Cells
The human immunodeficiency virus specifically targets and infects white blood cells known as CD4+ T cells, also called helper T cells. These lymphocytes coordinate the immune response, recognizing and signaling other immune cells to combat infections. They activate B cells to produce antibodies and stimulate cytotoxic T cells to destroy infected cells, orchestrating the body’s defense against various pathogens.
The CD4 protein on the surface of these T cells acts as a receptor, allowing HIV to bind and enter the cell. This selective targeting makes CD4+ T cells the main victims of HIV infection, as their destruction directly impairs the immune system’s ability to mount an effective response. A healthy immune system typically maintains a CD4 count between 500 and 1,600 cells per cubic millimeter of blood. In HIV infection, this count can drop significantly, and their depletion directly measures the immune damage caused by the virus.
How HIV Destroys Cells
HIV employs multiple mechanisms to destroy CD4+ T cells. One direct method is viral replication, where the virus rapidly produces new copies within the infected cell until it bursts, a process known as lysis. Each infected cell becomes a factory for new viral particles that then infect more cells.
Beyond direct lysis, HIV induces pyroptosis, an inflammatory form of programmed cell death in both infected and uninfected CD4+ T cells. This process involves cell swelling and membrane rupture, releasing pro-inflammatory molecules that contribute to chronic inflammation. Additionally, infected CD4+ T cells can fuse with uninfected cells, forming large, multinucleated cells called syncytia. These syncytia are dysfunctional and quickly die, leading to the demise of multiple cells through a single infected cell.
The persistent presence of HIV also leads to bystander killing. Uninfected CD4+ T cells die due to the chronic immune activation and inflammation triggered by the virus. Even cells not directly infected can be caught in this inflammatory environment, leading to their premature death. This combination of direct viral effects and indirect immune responses contributes to the progressive destruction of CD4+ T cells.
Weakening the Body’s Defenses
The continuous destruction of CD4+ T cells by HIV directly compromises the body’s immune system. As the number of these coordinating cells declines, the immune system loses its ability to effectively recognize and combat invading pathogens. This reduction in CD4+ T cell count leaves the body vulnerable to infections that a healthy immune system would typically manage without difficulty.
The compromised immune function manifests as an increased susceptibility to opportunistic infections. These are illnesses caused by microorganisms that usually do not cause disease in people with healthy immune systems but take advantage of weakened defenses. Examples include certain types of pneumonia, fungal infections, and specific cancers that are rare in the general population. The occurrence of these specific infections, alongside a low CD4 count, is often used to diagnose acquired immunodeficiency syndrome (AIDS), the most advanced stage of HIV infection.
Without effective treatment, the body becomes progressively less able to defend itself against these common and uncommon pathogens. This leads to recurrent and severe infections, which can spread throughout the body and become life-threatening. The vulnerability extends to various types of pathogens, including bacteria, viruses, fungi, and protozoa, highlighting the broad impact of CD4+ T cell depletion on overall immune competence.
Systemic Effects Beyond the Immune System
While HIV primarily targets the immune system, its long-term presence and the chronic inflammation it induces extend its destructive impact to other organ systems. Persistent low-level inflammation and viral presence in various tissues can lead to dysfunction over time, contributing to accelerated aging processes in many organs.
For instance, HIV can affect the brain and nervous system, leading to neurocognitive disorders such as memory loss, difficulty concentrating, and motor skill impairment. The virus can directly infect brain cells, and the chronic inflammation further contributes to neuronal damage. Additionally, individuals living with HIV often experience an increased risk of cardiovascular disease, including heart attacks and strokes, due to inflammation and metabolic changes induced by the virus and some treatments.
Beyond these, the kidneys can also be affected, with HIV-associated nephropathy being a specific kidney disease. Bone density can decrease, leading to conditions like osteoporosis and an increased risk of fractures. These effects are distinct from the initial immune cell destruction but represent the broader, long-term consequences of living with HIV, underscoring its widespread impact on overall health.