Common site of regional enteritis is -
Crohn disease (Terminal ileitis or regional enteritis) 

Crohn disease (CD) is an inflammatory bowel disease, characterized by -


Sharply delineated and typically transmural involvement of the bowel by an inflammatory process with mucosal damage.
The presence of non-caseating granulomas.
Fissuring with the formation of a fistula.


Any portion of intention can be involved but most commonly small intestine is involved.

Small intestine → 40%
Small intestine + colon → 30%
Colon → 30%

Terminal ileum involved most commonly and rectum is usually spared.
CD is associated with HLA-DM1/DQw5 and NOD 2 genes and an abnormal T-cell response, particularly CD4 + T cells (Th1 cells)
Colon
Rectum
Distal ileum and colon
Caecum