We're talking about a multicellular organism. An animal's skin keeps out things that its individual cell membranes aren't concerned with. (I don't actually thi

We're talking about a multicellular organism. An animal's skin keeps out things that its individual cell membranes aren't concerned with. (I don't actually think the details of what cell walls, cell membranes, skin, etc., do and don't regulate / completely block really tell us anything much about what should happen at national borders. If tomorrow someone discovered a previously unknown regulatory mechanism in cell membranes, that shouldn't make anyone change their mind about immigration controls. The way these analogies actually work is that someone decides what they think about immigration on other grounds, and then goes looking for an analogy that fits the rhetorical point they want to make. Someone arguing for "open borders" would probably choose something else rather than cells to make analogies with. The analogy isn't doing any real intellectual work, it's just window-dressing.)