Introduction:
Antigen-presenting cells (APCs) are a vital component of the immune system that play a crucial role in initiating and regulating immune responses. They capture, process, and present antigens to T cells, thereby activating the adaptive immune response. While most immune cells have some degree of antigen-presenting capabilities, some cells are more efficient in this function than others.

Explanation:
Among the options given, NK cells (natural killer cells) are the only cells that do not behave as antigen-presenting cells. Here's an explanation of why each of the other options can function as APCs:

1. Macrophages:
- Macrophages are highly efficient APCs.
- They have the ability to phagocytose pathogens and process the antigens derived from them.
- Macrophages express major histocompatibility complex class II (MHC-II) molecules on their surface, which are essential for antigen presentation to T cells.

2. Dendritic Cells:
- Dendritic cells are considered the most potent APCs.
- They have specialized receptors that allow them to capture and internalize antigens from pathogens.
- Dendritic cells process antigens and present them on MHC-II molecules, effectively activating T cells.

3. B-lymphocytes:
- B-lymphocytes are primarily involved in humoral immune responses mediated by antibodies.
- While their main role is to produce antibodies, B-lymphocytes also function as APCs in certain contexts.
- They can capture antigens through their B-cell receptors and present them to T cells, promoting T-cell activation.

4. NK cells:
- Unlike the other options, NK cells are not professional APCs.
- NK cells are part of the innate immune system and are primarily involved in eliminating virus-infected cells and tumor cells.
- While they can interact with other immune cells, NK cells do not have the specialized antigen presentation machinery seen in APCs.

Therefore, the correct answer is option 'D' - NK cells, as they do not behave as antigen-presenting cells.