Final answer:
Circulating pruritogens can include hormones, immune cells, and inflammatory cytokines, which are responsible for the sensation of itching. Macrophages are among the immune cells that can release pro-inflammatory chemicals which act as pruritogens.
Step-by-step explanation:
Circulating pruritogens refer to substances in the bloodstream that can cause the sensation of itching, known as pruritus. These can be a variety of substances such as hormones, immune cells, and particularly inflammatory cytokines. For example, mast cells release histamines and other chemicals that can act as pruritogens during an inflammatory response to an injury or an allergic reaction, as illustrated in Figure 42.5 from the provided reference material. In the context of immune responses and hypersensitivity, certain immune cells, like macrophages, play a dual role in both phagocytizing pathogens and releasing pro-inflammatory chemicals, as indicated in the given description of the body's response to a cut.