This type is extranodal - mostly happening in the lining of the stomach among individuals with chronic inflammation, like those with Helicobacter pylori infection, a bacteria that causes chronic gastritis.
In particular, certain pain medications used to reduce inflammation in joints have been discovered to work with H. pylori to create more severe stomach ulcers.
However, we do know individual genetics, other medical problems, use of certain medications, smoking, and the genetic diversity of Helicobacter strains all play a role.
Some of the proposed reasons are flatworm and hookworm infection, H. Pylori infection, gastrointestinal bleeding and their vitamin intake being disrupted by the incorporation of modern high-starch foods to the diet.