That would have more to do with the circulatory system, I think. Has the person been tested for anemia?
I dunno. I will ask, and I will return, hopefully within a week because thanksgiving interrupts the flow of work. :T
Also, to add further information, this person apparently gets sick often and has to take strong anti biotics.
I personally think that the doctor was wrong on this because I highly doubt a poor immune system could cause fainting from disease rampaging in the body constantly with all of the anti biotics taken.