The question on Quora was:

Why don’t I turn into a fish when I eat fish? Or a cow when I eat beef?

The expanded explanation for the question was:

Before this is labelled as silly, here is explanation: there are specific chemicals in our bodies whose purpose is to prevent foreign genetic material in a body and cell from being read and translated into mRNA. What are the names of these chemicals and processes? How do they work?

My answer was:

Not a silly question at all, especially if we bypass the “eat” part and ask why just, say, injecting a fish cell into our arm wouldn’t turn us into a fish, or why giving our kid a transfusion of mongoose blood wouldn’t turn him into a speedy superpowered whizzer.

There are two parts to the answer.  First, DNA needs to have an elaborate support system before it can do anything.  It needs to be in a nucleus, wrapped in histones and other proteins, with access to polymerases and other enzymes, and so on and so on.  Just shoving DNA into your bloodstream doesn’t give it access to those things, so it can’t take over your cells to make fish or mongoose type proteins.

(But if you add to the DNA a system for entering a new cell and taking over the built-in systems, then you have a virus, which essentially does exactly that — turns your cells into a system for making more viruses.)

But why not a whole cell? Why can’t a fish cell settle down comfortably into our body and just pump out fish proteins all day long?

The answer is the immune system, which is aimed at identifying things that are not “self” — that is, that don’t match the template of our own cells — and to attack and reject them.  The immune system is very good at that, and can even reject very similar cells, like those of a different human, so identifying a fish or a mongoose cell is very easy to do, and those cells would be very quickly attacked and destroyed.

(Today, the immune system is tuned for identifying and destroying bacteria and viruses and so on.  But it’s possible that hundreds of millions of years ago, much of the immune system was tuned for exactly what we’re talking about here — preventing foreign cells from moving into the friendly environment and taking over.)

In response to a question in the comments (“What about plants and simpler organisms which don’t have complex immune systems?”), I added:

You have to get very “simple” in the simpler-organisms category before they lose the parts of the immune system that recognize different cell types, although the actual mechanisms are quite different in different organisms.

As for plants, they don’t reject foreign material, at least not in the same way that animals do; you can graft different plants, or even different species, together and they’ll grow pretty happily.  Grape vines and apple trees, to name just two, often have this done.

As to how, or even whether, plants reject a more subtle invasion by parasitic plants … I just don’t know.  Hopefully a botanist can answer.