Explain how the immune system can communicate with the nervous system, how such communication can be reciprocal, and how these relate to stress and illness.

A. The immune system's cells secrete chemical messengers called cytokines.
1. One class of cytokines is proinflammatory; i.e., they promote inflammation.
B. Cytokines communicate with the brain.
1. This communication is thought to be via the peripheral nervous system.
C. The interconnection between the immune and nervous systems enables reciprocal or two-way interactions between both, i.e. each can influence the other.
D. This interrelationship may also enable common symptoms of illness, like fatigue and depression, to affect behavior.
E. Behavioral responses can activate processes affecting the immune system, as pointed out by Michael Irwin (2008).
F. Interactions between the nervous system and the immune system can result in symptoms related to stress and to illness.