Ileum

Reference
Pathologic features and clinical significance of "backwash" ileitis in ulcerative colitis. Haskell H, Andrews CW Jr, Reddy SI, Dendrinos K, Farraye FA, Stucchi AF, Becker JM, Odze RD. Am J Surg Pathol. 2005 Nov;29(11):1472-81. PMID: #16224214#

Collagenous ileitis is characterized by subepithelial collagen deposition in the terminal ileum, is an uncommon condition.
The few cases reported to date have been associated with collagenous colitis (CC) or lymphocytic colitis.
References
Collagenous ileitis: a study of 13 cases. O’Brien BH, McClymont K, Brown I. Am J Surg Pathol. 2011 Aug;35(8):1151-7. PMID: #21716082#