Cytokine

Cytokines are small proteins secreted by immune and other cells to communicate — recruiting cells to a site of damage, driving or damping inflammation, coordinating repair. Interleukin-6, TNF-alpha and the interleukin-1 family are among the best known.

It matters because muscle itself releases cytokines during exercise, known as myokines. Interleukin-6 from contracting muscle behaves quite differently from the same molecule in chronic disease: acutely it promotes an anti-inflammatory response and improves glucose handling, which is part of the mechanism behind exercise's systemic benefits.

In practice this is one of the clearest illustrations that context determines meaning in physiology — the same signalling molecule can be part of the problem or part of the adaptation depending on whether it spikes and resolves or stays chronically elevated.