I always believed there was no such thing as a mature immune system because it was always growing and changing. Please correct me if I'm wrong - my immune system may respond faster than my son's because I've "seen it before" but a vaccination that is new to both of us then we'll both have the same response from our immune system.

I'd rather get DS immunised for MMR because what if he got all three as a teenager and hung out with pregnant women? Brain damage, infertile AND loads of babies affected. OK, so not likely to happen but then again the side-effects are so mild I'd rather take that risk in my situation.