Suggest Remedy For Improving Immunity

Hi My mother has been getting cold n cough problem along with fever quite often for past 4 months.It seems her immunity is down.She is diabetic.(I took a random sample around 2 weeks back it was 278).This time she has throat infection,along with running nose and a slight fever.I have just given her paracetamol only.She was also diagnosed with Gall bladder stone which gave extreme pain in October 2012.Due to swellling in her common bile duct(CBD) her operation was deferred.In January Laproscopic surgeon told us there is still some swelling in CBD and it seems her recovery is very slow so doing operation right now will be dangerous as there are chances to internal bleeding.

Now considering her case history ,her catching Cold & throat infection quite often, should I assume this is something to do with gall stone.As her immunity is also down please suggest what should I give her in terms of medicine for improving immunity and also for throat infection.Further can this be due to pus being in her body that she is getting this cold n throat infection ??