Age is probably the biggest influence on a "healthy" immune system. I haven't kept up with recent research, but if memory serves the immune repertoire (diversity of T and B cell receptors) decreases essentially linearly with age.
I'm hopeful that cancer and immune profiling will be able to be used in tandem to (1) identify cancer early and (2) identify gaps in the immune repertoire to facilitate (3) design of antibodies that specifically target the cancer.
Here's some supporting research from a quick search. https://journals.aai.org/jimmunol/article/192/6/2689/1552
Edit: some fun but possibly pontificating tangents
This seems to be part of the reason why vaccines have had relatively poorer efficacy in elderly populations. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3901832/ https://www.nature.com/articles/s41586-021-03739-1
I'm hopeful that cancer and immune profiling will be able to be used in tandem to (1) identify cancer early and (2) identify gaps in the immune repertoire to facilitate (3) design of antibodies that specifically target the cancer.