How probiotics improve immunity and defend against viruses

A UCLA and Vanderbilt review (Ref.) of probiotics confirms that:

“One of the major mechanisms of probiotic action is through the regulation of immune response”.

That’s not surprising when over 70% of the immune system is located in the gut.

The right strains of probiotics have been found to enhance innate immunity in 4 main ways.

Probiotics can kill pathogens in the gut before they have a chance to cause harm (Ref.)
Probiotics directly improve the functioning of key immune cells T cells, B cells and macrophages (Ref.)
Probiotics strengthen the wall between the gut and bloodstream (the epithelial barrier), thereby preventing pathogenic microbes from spreading via the bloodstream (Ref.)
Probiotics enhance protection rates from vaccination (Ref.)

Reducing the risk and duration of respiratory illness

A review of 13 clinical trials (Ref.)  found that regular use of probiotic supplements is specifically effective for reducing the risk and duration of upper respiratory tract infections.