Hi Leen As long as your body gets its required nutrients, it will tend to produce the immune system to the best of its ability (taking in more than required will generally not give additional "bonus" as far as I know, despite what many producers of supplements etc. seem to indicate in their marketing. What is required might vary however, from individual to individual and through their lives in health and disease).
In situations of nutrient deficiency, your immune system can be weakened just like other functions of your body. E.g., famously deficiency of iron reduced production of red blood cells, and vitamins are needed to keep the immune system running.
Can I suggest asking your doctor to be more specific in her/his advice, and he/she thinks is lacking. You can also show him what Jake is, and get his opinion tailored for your context?
And, also: medical advice on the internet is usually not very good, and really really hard to interpret and apply to individual situations. Which is why a doctor that knows you is a better source of advice, or will point to some good ones.
(It's my personal belief that this kind of product - if well manufactured - might provide a good way to ensure your body gets what it needs, and be strong and environmentally friendly rival to our typical diets. But it's in some ways a new area, so one cannot be certain when it comes to really long-term use, thought it's a risk I am more than willing to take as I consider it quite small. Biology is however complex, and especially if you have a medical reason for the prescribed "boost", it's even harder to predict. Just remember than people online tend to be horribly binary [and zealous] in their views of nutrition, parting the world into "good" and "bad" - things generally arent that simple.)