Maybe so, but also maybe not quite my point, unless you know something I don't about it.

Sure, some samples will be off to antigen presentation, but does that inform more than this is an encountered foreign substance and this is how to bind to it for neutralization ? Seems like in principle you could take the overview picture and have something like olfaction, but for things that hadn't been sufficiently cracked open when they passed the olfactory epithelium. Maybe it's starting to be sorted out, but I'm not up to date on what the neural feedback from the immune system carries.

If the quantities are too low for the nose to pick them up I think nature has converged on "too noisy to bother with".