Is there anything genuinely useful for 'boosting immunity'?

Not in the way the supplement aisle suggests. There is no evidence that any product meaningfully raises immune function in a well-nourished person, and an over-active immune system would not be desirable anyway. What is actually supported: adequate sleep, since short sleep measurably reduces vaccine response and raises infection risk; correcting genuine deficiencies, with vitamin D and B12 being common in India and worth testing rather than guessing; not smoking; managing blood sugar, since poorly controlled diabetes clearly impairs immunity; and vaccination, which is the only intervention that actually trains the system.