In a startling discovery that raises fundamental questions about human behaviour, researchers at the University of Virginia School of Medicine have determined that the immune system directly affects – and even controls – creatures' social behaviour, such as their desire to interact with others.

So could immune system problems contribute to an inability to have normal social interactions? The answer appears to be yes.

"The brain and the adaptive immune system were thought to be isolated from each other, and any immune activity in the brain was perceived as sign of a pathology. And now, not only are we showing that they are closely interacting, but some of our behaviour traits might have evolved because of our immune response to pathogens," explained Jonathan Kipnis, PhD, chairman of UVA's Department of Neuroscience. "It's crazy, but maybe we are just multicellular battlefields for two ancient forces: pathogens and the immune system. Part of our personality may actually be dictated by the immune system."

The relationship between people and pathogens, the researchers suggest, could have directly affected the development of our social behaviour, allowing us to engage in the social interactions necessary for the survival of the species while developing ways for our immune systems to protect us from the diseases that accompany those interactions.

Social behaviour is, of course, in the interest of pathogens, as it allows them to spread.

The UVA researchers have shown that a specific immune molecule, interferon gamma, seems to be critical for social behaviour and that a variety of creatures, such as flies, zebrafish, mice and rats, activate interferon gamma responses when they are social. Normally, this molecule is produced by the immune system in response to bacteria, viruses or parasites. Blocking the molecule in mice using genetic modification made regions of the brain hyperactive, causing the mice to become less social. Restoring the molecule restored the brain connectivity and behavior to normal. In a paper published online in the journal Nature, outlining their findings, the researchers note the immune molecule plays a "profound role in maintaining proper social function."

"It's extremely critical for an organism to be social for the survival of the species. It's important for foraging, sexual reproduction, gathering, hunting," said Anthony J. Filiano, PhD, Hartwell postdoctoral fellow in the Kipnis lab and lead author of the study. "So the hypothesis is that when organisms come together, you have a higher propensity to spread infection. So you need to be social, but [in doing so] you have a higher chance of spreading pathogens. The idea is that interferon gamma, in evolution, has been used as a more efficient way to both boost social behaviour while boosting an anti-pathogen response."

The discovery that the immune system – and possibly germs, by extension – can control our interactions raises many exciting avenues for scientists to explore, both in terms of battling neurological disorders and understanding human behaviour.

"Immune molecules are actually defining how the brain is functioning. So, what is the overall impact of the immune system on our brain development and function?" Kipnis said. "I think the philosophical aspects of this work are very interesting, but it also has potentially very important clinical implications."