I am very interested in knowing some of the mecanism of the immune system function. For me it's important to keep it healthy and even if I always have been lucky enough to avoid any infections since I'm adult, I have some eating/sleeping habits, it's not extreme, I keep it in a balanced way...
Fruits and veg, preferably raw or slighly cooked to preserve the vitamins (though Vitamine A doesn't disappear with cooking, the issue is with Vitamin C)
I try to eat 5 fruits or veg a day but I'm not sure I really do so... some are good deals as they are both rich in vitamins and easy to eat (apples, parsley, clementines...) and I always wash them carefully to get rid of pesticides and I often buy them organic ( though I know organic is not always 100 % poison free, I think it goes in that direction and is always better than random...)
Supplements of vitamins don't provide vitamins in their right proportions whereas vitamins work in sinergy.. it has also been showed that taking to much of vitamin A can be harmful because it prevents some other less known nutriments from being well absorbed by the body... For the sake of precaution, I prefer to avoid taking those supplements.. I have never really expereinced them so I can't tell by experience but that's my feeling about it...
Sleeping well and enough, as the cells get reparated during the sleep and drinking water enough to get rid of toxins
Also beware that sometimes, when colds come often, or dry cough, it can be due to a breath allergy... the breath allergy simply make your mucous membrane weaker, which has nothing to do with immune system function... the syptoms of an allergy cold are a bit different from a virus/bacte cold, it doesn't really hurt but eaches a lot... however, it is an open door for viruses when the membrane is deteriorated and can lead to a "normal" cold, even with a decent immune system...