"Immune dysregulation sits at the root of an enormous range of human diseases, from autoimmunity to cancer to neurodegeneration," said Ramnik Xavier, a co-senior author of the study, a core institute member at the Broad Institute and Kurt Isselbacher professor at Massachusetts General Hospital and Harvard Medical School. "What excites me about this work is that it doesn't just tell us that a gene matters; it tells us the molecular mechanism, which can point toward therapeutic strategies."

A 'shock absorber' for vital genes

The single-cell data also revealed something unexpected about the genes most critical for health and survival. These genes are wired to dozens of regulatory elements at once, each contributing only a small change in expression. That redundancy keeps the genes' activity stable — and it explains why traditional studies, which look only at overall gene expression, have struggled to detect them. A disease-relevant switch may shift expression too slightly for such studies to notice.

"We think the body wants precise control over the expression of its most important genes; it doesn't want any single genetic variant to be able to swing that expression too far on its own," said first author Masahiro Kanai, a postdoctoral scholar at the Broad Institute and an instructor in medicine at Massachusetts General Hospital and Harvard Medical School. "So instead of relying on one strong switch, these genes are controlled by many weaker ones. We call this regulatory buffering, because it dampens the effect of any single chromatin change before it reaches gene expression, like a shock absorber."

From signal to mechanism

The team focused on immune cells because immune dysregulation underlies many diseases that look unrelated in the clinic. Across eight broad immune cell types, they ran the full analysis for thousands of disease-linked genes and traced the precise mechanisms behind several variants.

One variant, inside a gene called TNRC18, had been flagged in an earlier 2023 FinnGen study as linked to inflammatory bowel disease, with no known mechanism. The new data show it reduces TNRC18 expression in T cells — most strongly in a subset called T helper 1 cells — pushing those cells toward a more inflammatory state.

Another example involves the genes IL4R and IL21R. The data help explain why one variant in that region lowers asthma risk while a neighboring variant raises the risk of autoimmune hypothyroidism. The thyroid-risk variant turns up IL21R, the receptor for an immune signal called IL-21. Mice lacking that receptor on their T cells are protected from the same thyroid complication — a hint, the authors note, that blocking IL-21 might treat a similar thyroid side effect seen in some cancer immunotherapy patients.

Cautions and next steps

The researchers have made their data and methods freely available. They caution, though, that many of the mapped connections are still hypotheses rather than confirmed mechanisms. The atlas relies on immune cells from blood donors — mostly healthy individuals — so the team is now working to profile immune cells taken directly from diseased tissue.

"One of the biggest gaps in human genetics has been between finding a disease-associated variant and understanding its function," said co-senior author Mark Daly, an institute member at the Broad Institute and founding chief of the Analytical and Translational Genetics Unit at Massachusetts General Hospital. "This atlas provides reliable evidence that we can now systematically connect genetic association signals to molecular switches affecting specific genes and cells at scale."

"This is a launching pad for future research," said co-author Daniel Graham, an institute scientist at Broad. "There's a lot of reason to be optimistic about the power of using this type of data to uncover mechanisms of disease."

Publication: Masahiro Kanai et al., Nature (2026). DOI: 10.1038/s41586-026-11078-2