Can proteins/peptides pass through the intestine?
Entire proteins can be transported across the epithelial cells of the digestive system.https://en.wikipedia.org/wiki/Transcytosis

This is common in neonatal infants aiding in immunity passed via proteins in mother's milk. It is much less common as you age. http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_aacids.html 

The process is termed "transcytosis" if you wish to search for whether this occurs for a particular substance. One such goup of proteins entering the body in this manner are prions. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5004172/