[TheSpiceIsLife]

Caffeine reduces immune response:

Caffeine, like other xanthines, also acts as a phosphodiesterase inhibitor.[141] As a competitive nonselective phosphodiesterase inhibitor,[142] caffeine raises intracellular cAMP, activates protein kinase A, inhibits TNF-alpha[143][144] and leukotriene[145] synthesis, and reduces inflammation and innate immunity.[145] Caffeine also affects the cholinergic system where it inhibits the enzyme acetylcholinesterase.

https://en.wikipedia.org/wiki/Caffeine

[ben_w]

Huh. I’d never have guessed that, given its use in (non-prescription) cold-and-flu medicine.