In school we learned that all ingested proteins are digested via denaturation and breaking up the larger macro-structure into peptides and amino-acids so small that they can be absorbed and which are then re-used by the body for what ever it needs these basic building blocks. This is supposedly the fate of all larger protein blocks and necessary as only small peptides and amino-acids can enter the blood stream.
While this may be the normal case for normal protein digestion, this model may turn out to be too generalised. For example, if we look at certain prion-diseases it seems possible that the causative agent for these diseases are infectious proteins.
Scrapie modes of transmission have been debated for many years. Although experimental transmission can take several forms, the natural transmission of scrapie horizontally between individuals occurs through direct contact between animals and through contact with environmental contamination (reviewed in Schneider et al., 2008). Scrapie is predominantly acquired through the oral route, and the placenta and amniotic fluid are the most common sources of oral infection, although fetal parts, feces, and milk have all shown infectivity (see Schneider et al., 2008).
From: K.S. MacLea: "What Makes a Prion: Infectious Proteins From Animals to Yeast", International Review of Cell and Molecular Biology, Volume 329, 2017, p227-276.
That large proteins are active via the oral route seems to contradict the basic principles of protein digestion. Questions that arise here: Do some proteins survive human digestion? Which proteins survive? How or why do they survive?
To boil that down: is the general model of protein digestion and absorption incomplete with regard to larger protein structures? Wikipedia simply states that
PrP found in infectious material has a different structure and is resistant to proteases, the enzymes in the body that can normally break down proteins.