**Core Concept**
AIDS (Acquired Immunodeficiency Syndrome) is characterized by a deficiency in a specific subset of immune cells, leading to compromised cell-mediated immunity. This subset of cells plays a crucial role in initiating an immune response against viral infections. The deficiency of these cells results in the hallmark feature of AIDS, which is the inability to control opportunistic infections.

**Why the Correct Answer is Right**
The correct answer is related to the CD4+ T lymphocytes, also known as T helper cells. These cells are essential for activating and coordinating the immune response, including the activation of B cells and cytotoxic T cells. In AIDS, the HIV virus targets and destroys CD4+ T cells, leading to their severe deficiency. This deficiency compromises the body's ability to mount an effective immune response, making it susceptible to opportunistic infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** While CD8+ T cells (cytotoxic T cells) are also involved in the immune response, their deficiency is not the primary cause of AIDS. CD8+ T cells are more focused on eliminating infected cells rather than initiating the immune response.
* **Option B:** B cells are important for producing antibodies, but their deficiency is not the hallmark feature of AIDS. B cells can still function in the absence of CD4+ T cells, albeit with reduced efficiency.
* **Option C:** Macrophages are a type of immune cell involved in phagocytosis and antigen presentation, but their deficiency is not directly related to the pathogenesis of AIDS.

**Clinical Pearl / High-Yield Fact**
A key feature of AIDS is the inverted CD4:CD8 ratio, where the number of CD8+ T cells exceeds the number of CD4+ T cells. This ratio is a useful diagnostic marker for AIDS and can be used to monitor the progression of the disease.

**Correct Answer:** C. CD4+ T lymphocytes.