## **Core Concept**
The question tests understanding of the immune system, specifically the classification of lymphocytes. Helper and cytotoxic T cells are crucial components of cell-mediated immunity. They are categorized based on their surface receptors and functions.

## **Why the Correct Answer is Right**
Helper T cells (CD4+ T cells) and cytotoxic T cells (CD8+ T cells) are both T lymphocytes. They originate from hematopoietic stem cells in the bone marrow and mature in the thymus. These T cells play central roles in cell-mediated immunity: helper T cells assist in activating B cells and cytotoxic T cells directly kill infected cells or produce chemical signals.

## **Why Each Wrong Option is Incorrect**
- **Option A:** B cells, or B lymphocytes, are primarily involved in humoral immunity through the production of antibodies. They are not helper or cytotoxic cells.
- **Option B:** Natural Killer (NK) cells are a type of lymphocyte critical in innate immunity, especially against viral infections and tumor cells. They do not require prior antigen exposure to kill cells and are not classified as helper or cytotoxic T cells.
- **Option D:** While macrophages and dendritic cells are antigen-presenting cells (APCs) crucial for initiating immune responses, they are not helper or cytotoxic T cells.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the role of CD4+ (helper) and CD8+ (cytotoxic) T cells in HIV infection. HIV targets and destroys CD4+ T cells, leading to immunocompromise. Understanding the distinct roles of these T cell subsets is vital for managing HIV/AIDS and other immune-related conditions.

## **Correct Answer:** C. T cells.