I don't know anything about the course, but a Master's will firstly set you up nicely for a PhD, if that's what you decide you want to do. Is it research-based or taught?
With a Master's you can get a job as a technician or research assistant; you could do this while working towards a PhD in many institutions. Obviously you can apply to all the desk-based jobs you could with a bachelor's degree - I can't really see how a Master's could disqualify you from anything you could do without it, unless you're just doing it because you can't be bothered to get a job (in which case, it's probably a bad idea). You can work in an NHS diagnostic department, and I think you can work for many pharmaceuticals companies too, although I think a PhD can be an advantage in industry. You could probably get a job working for a scientific or medical journal as well, although again I'm not sure a PhD wouldn't be an advantage.
I'm afraid I can't be absolutely sure any of this is right because although I'm keeping half an eye on the jobs market for immunologists, I'm doing it under the assumption that if I do apply for anything I'll have a PhD when I do; so I can't be sure that all the above applies to those with a Master's. I'd suggest you do some research of your own; look in places like NatureJobs and the NewScientist job section to see what's available.