Final answer:
The signaling molecule that regulates white blood cell formation and activation is called cytokine. Cytokines are chemical messengers that regulate immune responses and are involved in the differentiation and maturation of cells.
Step-by-step explanation:
The signaling molecule that regulates white blood cell formation and activation is called cytokine. Cytokines are chemical messengers that regulate cell differentiation, proliferation, gene expression, and cell trafficking to affect immune responses. Interleukins, a class of cytokines, play a crucial role in hemopoiesis and are involved in the differentiation and maturation of cells.