Step 1: Understanding the Concept:
The Human Immunodeficiency Virus (HIV) is a pathogen that targets the human immune system, eventually leading to Acquired Immunodeficiency Syndrome (AIDS) if left untreated. HIV specifically strikes immune cells that express a particular receptor protein on their outer membrane known as the CD4 receptor. This surface protein acts as the molecular keyhole that the virus exploits to gain entry into the cell.
Step 2: Detailed Explanation:
Let's look at the primary targets of HIV inside the human body:
1. The principal cells carrying CD4 surface receptors are a specialized subset of T-lymphocytes called T-helper cells (or $\text{CD4}^+$ T-cells).
2. T-helper cells coordinate the adaptive immune response by activating other immune cells, including B-lymphocytes (which produce antibodies) and Killer T-cells (which destroy infected cells).
3. Once HIV enters a T-helper cell, it converts its viral factory to replicate, destroying the host cell in the process. Over time, the rate of cell destruction outpaces the body's ability to produce new ones.
4. Other T-cells, such as Cytotoxic/Killer T-cells, carry CD8 receptors instead of CD4, meaning HIV cannot directly infect or destroy them. B-cells also lack the CD4 receptor profile.
Consequently, HIV causes a selective, severe reduction in the population of T-helper cells only. This matches option (A).
Step 3: Final Answer:
HIV causes a reduction of T-helper cells only.