Scientists have for years theorized that the immune system and the brain are more interconnected than previously thought, with findings of recent studies backing this hypothesis. For example, researchers recently discovered there is a physical connection between the immune system and the brain’s blood supply. Now, researchers have recently begun to find out that there may be a more psychological connection.
According to researchers at the University of Virginia (UVA) and the University of Massachusetts (UMass) Medical School, the immune system may directly affect social behavior in certain animals, such as mice. The finding could have enormous implications for people with autism-spectrum disorders or with schizophrenia. 
Jonathan Kipnis, professor of neuroscience at UVA in Charlottesville, said:
“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 behavior traits might have evolved because of our immune response to pathogens.
“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.”
Kipnis and his colleagues found that blocking one particular kind of immune molecule in a mouse’s brain caused abnormal behavior that vanished after restoration of the molecule. 
When this molecule, called interferon gamma, was genetically blocked, the rodents’ brains became hyperactive and specific areas of the brain that govern social interaction were unable to function properly. The affected mice – normally highly social creatures – interacted significantly less with the other mice.
The researchers say this means that the immune system, along with controlling whether or not we get sick, also appears to control our desire to interact with others.
And that, in turn, could mean that immune system problems might play a role in one’s inability to have normal social interactions. 
“Whether we like it or not, there is a piling up of evidence that the immune system has a major impact on brain function: The brain is not isolated from the rest of the body.”
Fascinatingly, researchers say proof that immunity and social interaction are closely related lies in the fact that infections spread rapidly when animals are in close contact. The immune system-social interaction theory has been tested in mice, rats, flies, and fish using publicly available gene-expression data.
Fascinatingly, the researchers theorize that immunity and social interaction are closely related due to the fact that infections spread rapidly when animals are in close contact. Using publicly available gene-expression data, the immune system-social interaction theory has been tested in mice, rats, fish, and even flies.
When mice or rats are housed together, for example, there is an increase in the genes that interferon gamma activates, whereas those same genes are decreased in socially isolated rodents.
Zebrafish and flies exhibited a similar pattern.
Tony Filiano, a postdoctoral associate in Kipnis’ lab, said:
“Our hypothesis is that interferon gamma might have evolved as an efficient way to control the anti-pathogen response when organisms are social.”
If the immune system and social behavior evolved alongside each other, this could explain the results of other autism studies. Earlier this year, one such study showed that some pregnant women with elevated blood levels of interferon gamma went on to have children with autism and intellectual disability.
While interferon gamma is needed for social behavior, the study suggests that too much of the molecule might shut off its activity.
The findings might also explain why some children with autism seem to become more social when they have a fever, as elevated levels of molecules such as interferon gamma accompany fevers.
Kipnis says that understanding the link between the immune system and social behavior might help scientists find drugs that treat autism. Clinicians might be able to administer the medication in the cerebrospinal fluid, rather than via the brain.