Final answer:
Among the options provided, neurons (A) are not part of the immune system. Dendritic cells, macrophages, and mast cells are all immune cells that play roles in the body's defense mechanisms.
Step-by-step explanation:
The cell that is NOT part of the immune system among the options provided is A. neurons. Neurons are the fundamental units of the nervous system, specifically responsible for transmitting information throughout the body. They do not participate in the immune response or function as immune cells. On the other hand, B. dendritic cells, C. macrophages, and D. mast cells are indeed components of the immune system. Dendritic cells and macrophages are professional antigen-presenting cells, which are critical for initiating the adaptive immune response. Macrophages can also phagocytize pathogens and produce chemicals that promote inflammation. Mast cells are involved in the allergic response and can release histamine and other mediators that affect inflammation.