Because of diseases like type 1 and type 2 diabetes, insulin is something we know quite a bit about.
So it's surprising that we've just discovered something brand new about it: insulin provides a necessary boost to our immune system in the fight against infection.
All of which means scientists might eventually be able to develop new ways of either boosting or dialing down the body's immune system. This could be used to produce better vaccines, or to handle the impact of inflammatory diseases.
"We have identified one of metabolism's most popular hormones, specifically the insulin signaling pathway, as a novel co-stimulatory driver of immune system function," says one of researchers, Dan Winer from the Toronto General Hospital Research Institute in Canada.
"Our work characterises the role of this signaling pathway in immune cells, mainly T cells, opening up avenues in the future to better regulate the immune system."
Those with type 2 diabetes, or a greater risk of type 2 diabetes, are either resistant to insulin or don't respond well to it – and the same lab team had already spotted a link to immune cells inside abdominal fat, and the pro-inflammatory chemicals they released.
It's that link that the researchers wanted to investigate further in their new study. They think there is a link between persistent, chronic inflammation, problems with the normal functioning of the body's immune system, and resistance to insulin.
The study used genetically engineered mice with T cells that were missing an insulin receptor, to mimic insulin resistance. Without the boost from insulin, the T cells were shown to be unable to fight off infections, including the H1N1 flu virus.
According to the researchers, it's likely that the insulin receptors act as a second push to the immune system, helping T cells to take up nutrients so it can properly deal with attacks from foreign bodies.
Of course we're just talking about mice models for now, and as useful as these animals are in figuring out how the human body might react, the same sort of response also needs to be observed in humans to confirm what's happening.
Still, the results are clear, and point to a critical role for insulin in the immune system. That could not only lead to better treatments and vaccines for a host of infections, but also help protect those at high risk of contracting diseases – people who are already resistant to insulin.
T cells are at the heart of many diseases, the researchers explain, so the better we can understand them at the cellular level, the better we can combat them.
"The link between insulin and the immune system is not obvious," says one of the team, Sue Tsai, from the Toronto General Hospital Research Institute.
"It is fascinating to learn that immune cells, which require energy and nutrients for proper functioning just like all other cells in the body, are also regulated by metabolic signals from insulin."
The research has been published in Cell Metabolism.