Excellent - I went back at 35 - already had 2 degrees (saddo !). It was the best thing I've ever done - 5 years of great fun and getting to think again - loved it ! (I funded by selling the house - ouch !).
Immunology ! That's good fun ! Go for it - it's so good to have the time to think and learn...
Where are you (?thinking of) going ? (Important for things like biomedical sciences if you are looking for jobs afterwards or thinking about medicine long-term).
Cheers,
Andy