New Scientist this week has an article on immune systems.
It seems they are as variable in age as heart age etc and can vary 10 years or more either side of chronological age.
As ever the best way to keep it low is to keep weight down, eat a good balanced diet and keep active. It has been found that calorie restriction is good for immune systems, and says that any one on the original 5;2 or 6:1 original fasting diet or 16:8 daily fast diet have a head start.