I've been on LDN for over 3 years now.
The website listed above will tell you everything you need to know. It is very important to go through one of the pharmacies listed on the site because compounding pharmacies not listed often do not give the correct/prescribed dosage or the fillers they use may cause problems.
I do believe LDN is the reason why I have dodged a lot of 'common' things - colds, flu, many allergies are gone, etc.
LDN is an immune system modulator. If your system is under or over active, it will attempt to put it in order. Supposedly (do not quote me please), it stimulates your t cell production by 300%. I can't remember where I got that figure from, but I promise I didn't make it up. I have not gotten a CD 57 test done because I believe the fact that I take LDN would throw my numbers off (by showing that I am healthier than I truly am because of the immune effect). It really kicks your immune system into gear.