Cytokines play a key role in stress response by mediating communication between which two systems?

• A. The nervous system and the immune system
• B. The digestive system and the circulatory system
• C. The endocrine system and the nervous system
• D. The respiratory system and the immune system

Answer: (A)

Cytokines are signaling molecules that bridge the immune system and the brain to coordinate how the body responds to stress. When stress occurs, immune cells release cytokines such as interleukins and TNF-alpha. These molecules travel to the brain, either by crossing the blood-brain barrier or signaling through the vagus nerve, where they act on the hypothalamus and help trigger the stress response via the HPA axis. This immune–nervous system dialogue explains why the nervous system and the immune system are the pair involved. Other system pairings don’t capture this specific messenger role, since cytokines are not the primary messengers between digestive and circulatory systems, and although endocrine signals also modulate stress, cytokines function as immune-to-brain messengers that shape neural responses.