In my book I described how infection causes sickness behavior (chap. 2), one route being through stimulation of vagus nerve that innervates various organs. The vagus then activates circuits in the brain that lead to extended sleep, social withdrawal, etc. I also mentioned that signaling works in the other direction as well: the vagus sends signals to various organs, including immune centers such as the spleen. This circuit can tamp down inflammation by inhibiting the production of the pro-inflammatory cytokine TNF. The mechanism for how this works was unclear, however. A new paper from the group of Kevin Tracey (Rosas-Ballina et al., 2011) fills in some of the missing detail, including a surprising finding. They describe how the vagus innervates the spleen and causes the release of the neurotransmitter acetylcholine. The surprising part is that the acetylcholine is made and released not by neurons but by lymphocytes – T cells in the spleen. This neurotransmitter then acts on macrophages to block TNF production. This is another example of how the nervous and immune systems share molecules in common, which enables neurons and immune cells to effectively talk to each other.