Which mast cell mediator is both preformed and produced in the late phase?

The correct choice is TNFα. This mediator is unique among the options listed because it is initially stored in granules of mast cells and can be released quickly during the early phase of an allergic response. However, unlike other preformed mediators, TNFα can also be synthesized and released during the late phase of an allergic reaction. This late-phase response is characterized by the recruitment and activation of various inflammatory cells, and TNFα plays a crucial role in this process.

In contrast, histamine is primarily a preformed mediator released immediately upon mast cell activation, often contributing to immediate allergic symptoms but not produced anew during the late phase. Leukotriene C4, while important in the inflammatory process, is not preformed in mast cells but rather synthesized only after mast cell activation, thus making it more relevant to the late phase than the early. Prostaglandin D2 is similar to leukotrienes in that it is synthesized upon activation rather than stored in preformed granules, limiting its role strictly to the late phase.

Therefore, TNFα stands out for its dual role, engaging in both immediate and later stages of the allergic response, which is pivotal in understanding the biphasic nature of allergic reactions.