https://phys.org/news/2018-10-reveals-gene-immunity-species.html

Previous work has shown that many genes in the innate immune response have rapidly evolved in vertebrates. This is thought to be caused by the relentless pressure of attack from pathogens like bacteria and viruses. These include genes that make cytokine and chemokine molecules, which act in a variety of ways—some are inflammatory molecules that alert the body to danger; others restrict a pathogen's ability to multiply and others induce cell death. They represent a successful host strategy to counteract rapidly evolving pathogens.

The team showed that these genes which have evolved rapidly across species, also have highly variable activity in different cells within an individual's tissue.