 The cause is not entirely clear , but is thought to be multifactorial . It has even been suggested that aphthous stomatitis is not a single entity but rather a group of conditions with different causes . Multiple research studies have attempted to identify a causative organism , but aphthous stomatitis appears to be non @-@ contagious , non @-@ infectious and not sexually transmissible . The mucosal destruction is thought to be the result of a T cell ( T lymphocyte ) mediated immune response which involves the generation of interleukins and tumor necrosis factor alpha ( TNF @-@ α ) . Mast cells and macrophages are also involved , secreting TNF @-@ α along with the T cells . When early aphthous ulcers are biopsied , the histologic appearance shows a dense inflammatory infiltrate , 80 % of which is made up of T cells . Persons with aphthous stomatitis also have circulating lymphocytes which react with peptides 91 – 105 of heat shock protein 65 @-@ 60 , and the ratio of CD4 + T cells to CD8 + T cells in the peripheral blood of individuals with aphthous stomatitis is decreased . 
