"Immune boosting" is not a real physiological target. What research can show is whether a nutrient deficiency impairs immune function, and whether correcting it helps — a much narrower and more useful question.
Immune function, infection duration and defence-related nutrients.
15 foods, grouped by the strongest rating any of its claims in this area reaches. Ratings describe the research, not the food. How ratings are decided.
Early human work, or mostly lab and animal research.
Gallus gallus domesticus
A small, soft organ meat with a milder flavour and finer texture than beef liver, which is why it dominates pâtés and parfaits.…
Immune function: Contains compounds associated with normal immune function, notably vitamin A, zinc and B12. The relevant human research concerns correcting deficiency in these nutrients rather than the food itself.
Allium sativum
The bulb of Allium sativum, used raw, crushed or cooked as a seasoning in most cuisines. The USDA reference entry is per 100 g of…
Immune function: Evidence is limited for garlic and the common cold: a very small number of trials have looked at cold frequency and duration with garlic supplements, and reviews describe the pool as too small and too poorly reported to draw a conclusion from.
Actinidia deliciosa
A fuzzy-skinned berry from a woody vine, with green or golden flesh and small edible black seeds. It contains actinidin, a…
Immune function: Kiwifruit is a concentrated vitamin C source, and a small number of trials report shorter or less severe upper respiratory symptoms in older adults and in people with low vitamin C status. The trials are few and the effect in well-nourished people is unclear.
Citrus x sinensis
The sweet orange, a hybrid citrus grown commercially on every inhabited continent. Most of the world crop is pressed for juice…
Immune function: Vitamin C at supplemental doses has been studied extensively for cold duration, with small and inconsistent results in the general population. Trials of oranges as a food rather than vitamin C as a supplement are essentially absent.
Crassostrea virginica
Filter-feeding bivalve molluscs, farmed and wild-harvested, whose flavour tracks the water they grew in closely enough that…
Immune function: Contains compounds associated with normal immune function, principally zinc. The human evidence concerns zinc status and supplementation rather than oysters as a food, and applies mainly to people whose intake is inadequate.
Lentinula edodes
A cultivated wood-decay fungus native to East Asia, sold fresh and dried. Drying concentrates its flavour compounds and is how…
Immune function: A small number of trials report changes in immune cell markers after several weeks of eating shiitake. Those are laboratory measures rather than infection outcomes, and the injectable lentinan used in cancer care is a different thing entirely from eating mushrooms.
Not enough human research to say anything useful yet.
Capsicum annuum
Non-pungent cultivars of Capsicum annuum, harvested green or left to ripen to red, yellow or orange. Ripening changes their…
Immune function: Peppers are promoted for immunity because of their vitamin C content. Correcting a vitamin C deficiency does matter for immune function, but there is no dependable evidence that eating more peppers changes infection rates in well-nourished people.
Bertholletia excelsa
Large seeds harvested from wild Amazonian trees, notable for concentrating selenium from the soil to a degree no other common…
Immune function: There is not enough dependable human research to say that Brazil nuts change immune function. Correcting a selenium deficiency matters for immune competence, but that is a statement about the deficiency, not about the nut.
Concentrated floral nectar processed and dehydrated by honeybees, composed overwhelmingly of fructose and glucose with small…
Immune function: Local honey is a long-standing folk remedy for hay fever, on the theory that it exposes people to local pollen. The few controlled trials do not support it, and the pollen carried in honey is largely heavy flower pollen rather than the wind-borne pollen that drives seasonal allergy.
Milk fermented with kefir grains, a rubbery symbiotic matrix of bacteria and yeasts that is strained out and reused. The yeast…
Immune function: Evidence is limited for kefir affecting immune function in humans. Much of what is cited comes from cell and animal work, and the few human trials are small with markers rather than infection outcomes.
Allium cepa
The bulb of a cultivated Allium, used as a base flavour in most of the world cuisines. Cutting the bulb releases enzymes that…
Immune function: Onions feature in a great deal of folk advice about colds and infections, including the belief that a cut onion in the room absorbs illness. None of that has dependable human research behind it.
Origanum vulgare
The dried leaf of Origanum vulgare, dominated by the phenols carvacrol and thymol. The USDA entry is per 100 g of dried leaf,…
Immune function: Oregano oil is widely sold for colds, sinus problems and infections generally. Controlled human trials are almost nonexistent; the antimicrobial data are laboratory work at concentrations no dietary or capsule dose achieves in the bloodstream.
Shredded cabbage salted and left to ferment under its own brine, where lactic acid bacteria already on the leaves acidify it…
Immune function: Evidence is limited for sauerkraut affecting immune function. The claim is widespread but rests on the general reputation of live cultures rather than on human trials using this food and measuring infection or immune outcomes.
Thymus vulgaris
A small-leaved Mediterranean subshrub used fresh or dried, and one of the few herbs that stands up to long cooking. Its…
Immune function: Thymol is strongly antimicrobial in laboratory conditions at concentrations far above anything eating thyme could produce in the body. No human research shows an effect on infection rates, so this remains a laboratory finding.
Agaricus bisporus
The most widely cultivated mushroom in the world, sold at several stages of maturity under different names. White button, cremini…
Immune function: Mushrooms are widely marketed for immunity on the strength of their beta-glucan content. Human trials using culinary mushrooms and measuring infection outcomes are almost non-existent, and laboratory activity of isolated polysaccharides does not carry over automatically.