Diagram 5: The immune system responds to the invasion

H. pylori use a specialised system to inject a
toxin called Cag A into epithelial cells. The cells
signal to the white blood cells of the immune system to
produce destructive oxygen compounds. H. pylori is
protected from these compounds by enzymes that break
them down and render them inactive. H. pylori strains that
do not make Cag A are less virulent than H. pylori that do.
Infection with Cag A strains also increases the risk of
developing cancer. This, and other genetic differences in
H. pylori strains, could explain why most people infected
with H. pylori do not develop ulcers or cancers.

The programmed cell death of epithelial cells — also called
apoptosis — that results from infection with H. pylori is
thought to be the main reason for the development
of cancer.