Well, the immune system is a really important thing, in my microbio class, the professor was really excited when he got to the immune system section.
The immune system is what kills any foreign pathogen like bacteria, virus, and funguses. when it's weak or doesn't work properly, u get sick. People don't die from flu, as the flu weakens ur immune system, what actually kills u when u get the flu is the secondary infection that follows which is usually pneumonia. a virus attacks and then a bacteria gang bangs on the immune system = x__X if u don't get treatment for the pneumonia infection.
Before antibiotics, people used forms of silver as it has antimicrobial properties. that is why you hear about silverware, which rich people could afford back then hence the rich survived the bubonic plague while the poor did not. soldiers in the civil war added real silver coins into a jug of milk to delay spoilage. There's a solution called colloidal silver which boosts the immune system and has antimicrobial properties and the interest in silver has gone up since it can get rid of MRSA while antibiotics are meeting resistance.
How do you know? well if you had allergies everyday in the morning and u have less or none anymore, then that's one way.
memory cells and killer T cells are abundant when there is an ongoing infection, an active one... i don't think you'll see an increase in these cells when u take probiotics or colloidal silver. Instead,the silver interacts more with the pathogens so a microscope would be a better way of measuring efficacy.
To me... the idea that your immune system attacks you is ridiculous, i would rather want to think that the pathogens do a form of mimicry that causes the killer T cells to attack the cells. of course more research should be done...
as for ur last question ,..is this some type of opinion question? The human body is dynamic, it's either you're at your highest potential or you're at a weaker level. if you are thinking of a situation of when it doesn't work at all...then you'll be dead in no time by the common cold, buddy