The etiology of Crohn’s disease is unknown, although two major hypotheses have evolved: an infectious and an immunologic theory. The following statement(s) is/are true concerning the possible etiology of Crohn’s disease

a. The leading infectious agent thus far suggested is infection with a Mycobacterium species

c. Although many alterations in cellular and immune functions in patients with Crohn’s disease have been observed, no primary defect in the immune system has yet been identified

Investigations for the last 60 years have tried to determine the etiology of Crohn’s disease. Although a number of theories have evolved and evidence is available supporting numerous theories, there is no conclusive evidence to support any etiologic theory for its development. Given the characteristic histologic findings of granuloma formation, early investigations focused on bacterial causes of Crohn’s disease, most notably infection with Mycobacterium species. Several reports have isolated Mycobacteria from mesenteric lymph nodes and intestine involved in Crohn’s disease but have not proven the Mycobacterial cause. Similarly, research in viral causes has been inconclusive, and although viral pathogens have been isolated from tissue extractions with Crohn’s disease, linkage to induction and persistence of the disease has not been convincing. Similar difficulties exist in theories concerning immunogenetic causes of Crohn’s disease, and although many alterations in cellular and immune functions associated with Crohn’s disease have been observed, no primary defect, either systemic or mucosal, humoral or cellular has been identified. A number of reports have described antibodies and lymphocyte reactivity to enterocytes, however the presence of antibody cannot be correlated with disease activity and furthermore antibodies have been found in patients with other diseases and in healthy volunteers.