How to strengthen my immune system

Anyone got any tips on how to strengthen my immune system ?

I’m always poorly with something. I’ve currently got sinusitis after having a cold for 2 weeks and I’ve had enough. I seem to always have something wrong either tonsillitis or the flu or a cough or I genuinely feel weak and run down and I’m sooo tired of being poorly.

I catch illnesses so easily its unreal and I want to change it, I do get the flu vaccine every year as I’m a severe asthmatic but it doesn’t really help.

Any tips and advice welcome !