**Core Concept**
The human immunodeficiency virus (HIV) primarily targets and infects cells of the immune system, including macrophages. This process involves specific receptors on the surface of these cells. **CD4** receptors are crucial for HIV entry into host cells.

**Why the Correct Answer is Right**
HIV binds to macrophages that express **CD4** receptors. The virus uses its envelope protein **gp120** to bind to **CD4**, which is a critical step in the viral entry process. This interaction is facilitated by co-receptors such as **CCR5** or **CXCR4**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while other receptors may be involved in the immune response, they are not the primary receptors for HIV binding to macrophages.
**Option B:** Incorrect as it does not specify the correct receptor involved in HIV binding.
**Option C:** Incorrect because, although related to the immune system, it is not directly involved in the primary binding of HIV to macrophages.

**Clinical Pearl / High-Yield Fact**
Remember, **CD4** count is a critical marker for monitoring the progression of HIV infection and the effectiveness of antiretroviral therapy.

**Correct Answer:** D. CD4.