I doubt that there are any magical, external things you can do or take that will boost the immune system anymore or any better than your body can...

... So essentially that simply means taking care of your body with good nutrition (with vitamins, nutrients, minerals via mostly fruits and vegetables -- and meat has some too) and adequate sleep...

I know I am far more likely to get sick shortly after some major project that has kept me stressed out, not eating right and not getting enough rest -- where I simply over-extended myself... The body has to build all those macrophages and neutrifils and T-Cells and such and if it doesn't have the right nutrients or it is spending all its energy on some project then it cannot build and maintain the myriad pieces of its immune system.

There are so many misconceptions about the immune system: Like "Cold gives you a cold"... Like many "truths", that is partly true. If you go out without your hat and gloves and boots in cold, snowy weather, your body may become overly chilled and have to divert resources from things like the immune system in order to deal with the hypothermia...

In short, listen to your mother! Eat your vege's, get to bed on time, and wear your boots and raincoat!