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.

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.

There's a huge variability in the age of thymic involution. Some people have a relatively active thymus at age 50 or 60, whereas in others it has switched off by age 25 or 30.

For those not familiar with immunology, the thymus is what populates your immune system with brand new T cells with freshly selected T cell receptors.

This suggests interesting anti-ageing opportunities, such as artificial thymi to repopulate your immune system and keep a healthy and diverse repertoire.

I wasn't aware of the variability in thymic involution. I guess most studies focus on averages. If you have links to more information about these outliers I'd be very interested in reading up on them.

The most recent literature that I'm seeing is showing the most pronounced time of thymus involution occurring around puberty, suggesting a programmed switch from a growing to reproductive phase of life.

"Regardless of the seemingly crucial role of the thymus in preserving homeostasis, its involution in humans and other mammals begins in childhood and peaks around puberty, resulting in an almost completely non-functional organ in aging." https://www.sciencedirect.com/science/article/pii/S156816372...

In this light the following makes sense (stopping or suppressing the switch to a reproductive life). "Castrating rodents before puberty or reducing the levels of sex hormones [e.g., by using Lupron, which desensitizes the luteinizing hormone-releasing hormone (LHRH) receptors] can attenuate or markedly recover the involution process in aging mice."

This study presents a gene expression marker for recent thymic emigrants and shows variability increases with age: https://insight.jci.org/articles/view/93739/figure/5

A much older study with different markers shows something similar: https://www.pnas.org/doi/epdf/10.1073/pnas.070061597

unrelated but spurred by this comment, have you seen studies exploring correlations between covid infection/severity and tonsils? a cursory google search yielded nothing.

See BRCA1 mutation