How does HIV primarily affect the immune system?

HIV primarily affects the immune system by destroying CD4 cells, which are a type of T-cell crucial for the immune response. CD4 cells help coordinate the immune system's response to infections by signaling other cells to engage and attack pathogens. When HIV enters the body, it specifically targets and infects these CD4 cells. Once inside, the virus uses the cell's machinery to replicate itself, ultimately leading to the death of the infected CD4 cell.

As the number of these cells declines, the immune system's ability to fight off infections and diseases diminishes, making individuals more susceptible to opportunistic infections and certain cancers. This progressive loss of CD4 cells is a hallmark characteristic of HIV infection, which can lead to a clinical stage known as AIDS if left untreated.

The other options do not accurately reflect the mechanism through which HIV impacts the immune system: enhancing immune function is contrary to the effect of HIV, increasing the number of T-cells does not happen since CD4 cells decrease, and preventing infections from occurring is not possible due to the compromised immune response. Therefore, the correct understanding of HIV's impact on the immune system is centered on the destruction of CD4 cells.