Here are the links to various universities and scientific journals.
According to Harvard University Health, "the best way to get enough Vitamin D3 is taking a supplement because it's hard to eat enough through your food."
Zinc is known to play a central role in aiding the immune system.
In one randomized clinical trial, 60 subjects with flu-like symptoms who took elderberry extract saw symptoms clear up 4 days earlier than placebo. Another clinical study showed positive results for helping clear up cold symptoms for air travelers.
According to the NIH, "Vitamin C deficiency results in impaired immunity & higher susceptibility to infections." Ensure you get your daily Vitamin C intake for a strong immune system.
Magnesium plays a major role in modulating and helping to control the immune response in a healthy manner.
Echinacea is an herbal flowering plant extract that is widely used for immune support all around the world.
Research shows that echinacea may help shorten the length & severity of colds, as well as upper respiratory infections (URIs).