microbiota and immune system

The combination of the host and its microbes is known as the 'holobiont' (mutualism). It is this co-dependence, this evolutionary inevitability, that led...to propose another level on which natural selection can act.

Tolerating some non-self substances and attacking some self molecules is just as important as destroying non-self pathogens.

People infected with hepatitis A or measles didn't have allergies as often as the uninfected...Strachan himself found that babies who had suffered an infection in the first month of their lives were no more likely to develop allergies than those who hadn't.

Professor Agnes Wold's reframing (再構造) of Strachan's original hygiene hypothesis has gained ground among scientists. Research on the body's microbiota have added our understanding of what triggers the immune system to develop in a healthy way. It is not simply infectious diseases that a super-clean environment puts a stop to, but normal colonization by the microbes that are sometimes referred to as 'Old Friends'. These old friends have evolved with us every step of the way, deeply engaged in conversation with the immune system as they have gone along.

Professor Sarkis mazmanian discovered a 'password', produced by a bacterium called Bacteroides fragilis. This species is one of the most numerous members of the microbiota, which often takes up residence in the gut immediately after birth. It produces a chemical called polysaccharide A (PSA), which is released from the surface of B. fragilis in tiny capsules. The capsules are engulfed by immune cells in the large intestine, and the PSA inside triggers the activation of regulatory T cell. The regulatory T cell then send out calming chemical messages to other immune cells, preventing them from attacking B. fragilis.

Antibiotics rarely decrease the total number of protective microbes present; instead, they shift the composition of the different species. It seems the real change is in how the immune system behaves, according to which members of the microbiota are present.

The drop in virulence (of Ebola), and mortality rate, means victims live just that little bit longer, giving the virus a better chance of switching to a new host and perpetuating (延續) its spread.

Collen, A. (2015) 10% Human: How Your Body's Microbes Hold the Key to Health and Happiness. London: William Collins.