Why does a person eat proteins in the form of meat, fish, eggs without dangerous consequences, and in no case is it possible

Why does a person eat proteins in the form of meat, fish, eggs without dangerous consequences, and in no case is it possible to inject proteins directly into the blood to feed patients?

1) Proteins in the digestive tract, in the stomach, in an acidic environment are broken down to amino acids by enzymes peptidases.
2) Amino acids already enter the bloodstream and are carried to tissue cells.
3) The introduction of foreign proteins into the blood will cause an immune reaction, rejection, and even death of the patient is possible.