T
he immune system is a complex network of white
blood cells comprising two units. The irst
encompasses the adaptive immune cells that
speciically recognise and respond to foreign
bodies, while the second unit comprises innate
immune cells that react in a more generic way, utilising alert
signals such as the chemical factors that signal inlammation
to kickstart their response.
Our body’s largest physical barrier is its epithelial surface.
It forms a protective “glove” separating our internal machinery
from the outside world. This barrier is formed by the skin on
the surface; internally, epithelial cells line the lungs and gut to
provide a protective layer.
Lying immediately beneath these barriers is a very intricate
network of innate immune cells known as innate lymphoid
cells. This family of cells is thought to play important roles in
protection of the gut, lungs and skin. Due to the strategic loca-
tion of these innate lymphoid cells at the body’s protective
barriers, they are ideally situated to sense signs of danger or
infection.Bacteria and Immune Cells in the Gut
The intestine performs the important function of digesting
the food we eat and converting it to energy. The gut also contains
more than 100 trillion bacteria from an estimated 500–1000
species that together weigh1.3 kg. Although we often think of
microorganisms as harmful, most of the bugs that inhabit our
body’s surfaces are essential for our health.
About 70% of our immune cells are localised to the gut.
This is unlikely to be coincidental – the close proximity of each
of these different elements suggests that the immune system
and microbiota might co-regulate each other.
When a pathogen breaches a body’s surface it generally
damages or disrupts the epithelial cells. This results in the release
of chemical mediators that activate innate immune cells. Thesethen recruit adaptive immune cells that mount a full assault to
eliminate the pathogen.
The adaptive immune cells are distinct from the innate cells
as they can distinguish different types of pathogens or threats
that need to be eliminated while leaving our own cells protected
from attack by the immune system. They do this by essentially
recognising the “signature” of these pathogens. The adaptive
immune cells can then mount an immune response that is
speciically tailored towards destroying the invading organism
and eliminating it from the body.
Over the past few years, immunologists have discovered a
number of novel populations of immune cells that have blown
apart some of our conventional views on how the immune
system works. We think it’s very unlikely that the innate
lymphoid cells are simply left over from evolutionary shifts,
and believe instead that they perform important and speciic
functions.
We wanted to try and discover what these roles might be and
how they contribute to health. This led us to establish a mouse
model of infection with the murine pathogen Citrobacter roden-28 | JUNE 2016
Guardians
of the Gut
LUCILLE RANKIN & GABRIELLE BELZThe appendix has long been considered an
evolutionary relic but new evidence indicates
it has an important role in our immune system.freshidea/adobe