FannyCornforth
Thank you Growstuff, yes, he has a lot of blood tests
I've had low white blood cell count a few times. It makes me more prone to infections. An immunologist isn't necessary and won't have any solutions. The next stage is to try and identify the cause of the low white blood cell count and treat that. You can't treat low white blood cell count with magic pills or supplements. I also try to avoid infection risks.
If he has symptoms of an auto-immune condition or is having cancer treatment, he might get referred to an immunologist.
Immunity caused by having viruses is something else and even know isn't that well understood.
If Mr C has had blood tests, presumably you've seen the results and you can Google what they mean and whether any action is necessary. Your GP should have informed him.