Jejunal immunofluorescence studies have shown that there is a significantly increased incidence of extracellular IgA and complement in the basement membrane zone and lamina propria of untreated adult coeliac patients compared to coeliac patients on a gluten-free diet. IgG was also demonstrated with complement, particularly in the untreated patients. These findings, taken together with those of other studies, suggest that a local antibody-antigen reaction involving IgA and complement may be responsible for the ongoing mucosal damage in untreated coeliac disease as well as for the acute damage following gluten challenge of the treated patient. Furthermore, consequent upon this damage there may be a secondary IgG antibody response, possibly reticulin antibody, contributing to the mucosal damage.