This is not something that suggests exercises and some other stuff. I know practically, those might not be possible to many, due to the busy schedules. But here are the few tips, which are even practically possible. Try them to strengthen your immune system.
Listen to your favorite music: It gives you mental health
Call your funniest friend, who can make you smile any time: It even gives social health along with mental.
No smoking please: Might be difficult to get rid of it. But it is possible, if you think of your dependant people.
Enough sleep: A deep sleep of at least 8 hours would strengthen your immune system.