An immune system that has been boosted is, in clinical terms, an immune system causing you problems. Autoimmune disease is an overactive immune response. What people actually want when they ask for an immune booster is to catch fewer colds and recover faster — a reasonable goal, and one that a handful of things genuinely help with.
Vitamin D
The strongest case on this list. Deficiency is common at higher latitudes between October and March, and correcting it reduces the rate of acute respiratory infection — most clearly in people who were deficient to begin with. Most health authorities recommend around 10µg daily through the darker months for most adults. This is the one supplement we suggest almost universally.
Zinc
Lozenges started within 24 hours of symptoms appearing shorten a cold by roughly a day in pooled analyses. The details matter more than the headline: it needs to be a lozenge that dissolves in the mouth, taken every few hours, at the first sign. A zinc capsule swallowed on day three will do nothing for that cold.
Do not take high-dose zinc continuously. Sustained intake above about 25mg daily interferes with copper absorption.
Elderberry
Reasonable small trials suggest a shortening of symptom duration. The effect sizes are modest and the studies are mostly small. We stock it, our customers rate it highly, and we would put it well below vitamin D in priority.
Sleep, and the unglamorous rest
People sleeping under six hours a night are substantially more likely to develop a cold after exposure than those sleeping seven or more. No supplement on our shelves has an effect of that magnitude. Handwashing, current vaccinations, and not smoking round out a list that is boring precisely because it works.
