The gene for a human protein is inserted into the genetic code of other cells that can produce a protein that is identical to what is made in the body.
The technology to program bacterial cells into producing human proteins has been around for decades and has revolutionized the way pharmaceutical companies are able to produce injectable proteins like human insulin, for example. The human gene for insulin has been inserted into a common strain of bacteria, cultured in huge tanks to produce insulin, which is then collected, purified and used to treat diabetes without getting many of the allergies associated with insulins derived from pigs or cattle (as was the case over 30 years ago).
Plants are more ideally suited to producing recombinant human Intrinsic Factor (rH-IF) because, unlike animal cells, plants don't have B12-binding proteins to interfere with the purity of IF produced by this technology. The rH-IF has been tested in humans and shown to work in a published pilot clinical study.