Which term describes cytokine signaling that travels through the circulatory system to affect distant cells?

• A. Endocrine
• B. Autocrine
• C. Paracrine
• D. Juxtacrine

Answer: (A)

Endocrine signaling is long-range communication. Signaling molecules like cytokines or hormones are released into the bloodstream and travel through the circulatory system to reach distant target cells that have the appropriate receptors. This fits the description of affecting cells far away from the signaling source. In contrast, autocrine signaling acts on the same cell that releases the signal, paracrine signaling targets nearby cells locally and typically doesn’t rely on the bloodstream, and juxtacrine signaling requires direct cell-to-cell contact via membrane-bound signals. Therefore, the term that describes cytokine signaling traveling through the circulatory system to distant cells is endocrine.