The human immunodeficiency virus (HIV) is an infection that targets and gradually weakens the body’s immune system. This viral attack makes it increasingly difficult for the body to fight off other infections and diseases. Understanding which specific cells HIV primarily targets is fundamental to comprehending how the virus progresses and causes illness.
CD4+ T Cells The Primary Target
HIV primarily targets a specific type of white blood cell known as CD4+ T cells, also called T-helper cells. These cells are a crucial component of the adaptive immune system, playing a central role in coordinating the body’s immune response against various pathogens like bacteria and viruses. CD4+ T cells do not directly attack infected cells or pathogens; instead, they act as messengers, signaling to other immune cells, such as B-lymphocytes and cytotoxic T cells, to activate their functions. This coordination is essential for an effective immune defense.
HIV specifically targets CD4+ T cells due to a protein called the CD4 receptor on their surface. This CD4 receptor acts like a specific “keyhole” that the virus uses to initiate its entry into the cell, allowing HIV to bind to these helper cells and making them vulnerable to infection.
How HIV Enters Its Target Cells
The process by which HIV gains entry into CD4+ T cells is a multi-step molecular interaction. It begins with the virus binding to the CD4 receptor on the T-helper cell. This initial binding causes a change in the shape of the viral envelope protein, exposing a site for secondary receptors, known as co-receptors, primarily CCR5 and CXCR4.
The virus requires both the CD4 receptor and one of these co-receptors (either CCR5 or CXCR4) to fully attach and fuse with the cell membrane. Interaction with the co-receptor leads to conformational changes that facilitate the fusion of the viral membrane with the CD4+ T cell’s membrane. Once fusion occurs, the viral genetic material and associated enzymes are released into the host cell’s cytoplasm, marking successful entry.
The Immune System Under Attack
When HIV enters a CD4+ T cell, it takes over the cell’s machinery to replicate, producing thousands of new viral copies. This process damages and destroys the infected CD4+ T cell. The continuous destruction of these helper cells leads to a gradual weakening of the entire immune system.
This compromised immune state leaves the body susceptible to infections that a healthy immune system would normally fight off. These are known as opportunistic infections, caused by various germs like bacteria, viruses, fungi, and parasites. Examples include certain forms of pneumonia, candidiasis, and toxoplasmosis. The body also becomes more vulnerable to certain cancers.
Stages of HIV Disease Progression
The progression of HIV infection is characterized by three main stages. The first stage is acute HIV infection, which occurs within 2 to 4 weeks after initial exposure. During this period, the virus rapidly multiplies and spreads throughout the body, leading to a sharp, temporary drop in CD4+ T cell count, and some individuals may experience flu-like symptoms.
Following the acute stage, the infection enters clinical latency, also known as chronic HIV infection. In this stage, the virus continues to reproduce in the body, but at much lower levels, and individuals may not experience significant symptoms for many years. Despite the lack of symptoms, the human immunodeficiency virus continues to slowly deplete CD4+ T cells, gradually weakening the immune system. Without treatment, this chronic stage can last for about 10 years.
The final stage of HIV infection is acquired immunodeficiency syndrome (AIDS), which develops when the immune system is severely damaged. AIDS is diagnosed when the CD4+ T cell count falls below 200 cells per cubic millimeter of blood, or when certain AIDS-defining opportunistic infections or cancers appear. At this point, the body’s defenses are profoundly weakened.