After C. diff., I had a lot of immune systems issues: pneumonia 4X, constant UTI's. etc. I, too, have IC so together with IBS, acid reflux, and IC, it's dificult to know what to eat. After years of frustration (writing a food diary, etc.), I was sick of the whole thing (as well as being physicallly ill). I developed a "Anything in moderation, nothing to excess" policy. Some days I am miserable; some days I am fine. I am on 50 mg Macrobid for the IC (plus a hormone cream) and would like to get off of it. Macrobid was previously considered "safe" but now has C. diff. warnings.
Years ago, I went to an immunologist who told me what bad shape I was in and how I would wind up on IVGG, and my insurance policy probably wouldn't pay for it. Didn't return for four or five years when he told how much better I was doing. (After drawing vials more of blood.) I told him off (including reminding him to wash his hands between patients) and never went back. (This was supposedly the "best"immuno. in the area. He did nothing for me. Perhaps "time" - and staying away from him helped.)
I believe what has helped me keep from "catching" stuff is good hygiene. I wash my hands whenever necessary (have become less of a "germ nut" lately, however) and keep a bottle of alcohol wash in the car. I use it after shopping -- not as good as soap and water, but who can carry it around? I also have a "thing" about using my own pen (have seen so many sales associates, etc. "stick a pen" in the mouths.). I also do saline irrigation (see FAQ) twice a day. It is suposed to "wash out" some of the bad bacteria we inhale during the day. I am in crowds (airports, movies, church etc) a lot and sometimes wear a portable air purfier (available on the internet - really don't know if it's helped or not --- only use it if I am "stuck" next to someone who is obviously ill). I try to stay away from ill people -- if possible. Make sure to get your flu shots plus a pneumovax shot -- but not if you are ill at the time.
You might try some of the "immune" meds. - but only with your doc.'s OK as many meds. (even supplements) can have contraindications.) I have used Echinachea in the past with good results. It is popular in the UK. Once, while visiting my son and his family there, everyone was sick with resp. infections. I started the Ech. and was the only one who didn't get ill. It's to be used sparingly, however, and not on a constant basis. Check it out in a health food store (a reputable one) with a reputable naturopath, and your own doc.
Good luck to you. Keep us posted.