I asked ChatGPT if it is OK to an explain a joke without being asked to do so, and She said no.

To deal with:

1. Bacteria

2. Parasites

3. Cancer

In all these cases cells that are "other than your own" are in your system and need to be eliminated, in the case of cancer it's your own cells but with significant mutations, as far as I can tell there is always an immune response although in some cases it is actually counter productive.

There are extremely good security reasons for it.

Even if we assume your body has a foolproof is_same_species() test, your neighbors' cells may:

1. Be healthy, but have the wrong priorities or behavior, so that they start doing the wrong tasks or sending the wrong messages in the wrong places. [0]

2. Be healthy, but they're just a creepy flesh-mask a parasite has constructed around itself.

3. Be individually hijacked by viruses, and your body won't have the same baseline information to tell that they've deviated from your neighbor's standard.

4. They may be fully human, but gone feral into a transmissible tumor [1].

____

[0] This is actually a risk from pregnancy, as overambitious fetal cells leak into the mother.

[1] https://en.wikipedia.org/wiki/Clonally_transmissible_cancer

The way your body detects foreign cells is the same way your body detects foreign cells. It detects anything that doesn't have your, specific, randomly-generated at conception, friend-or-foe markers.

I'm a kidney transplant patient myself. Your cells have HLA markers to distinguish your own cells from foreign cells. That is to prevent your immune cells from attacking your own body. This also makes it harder to find a organ match.

The reason I needed a kidney transplant is because of an auto disease called IgA Nephropathy where the IgA antibody has a small defect that causes the rest of the immune system to attack it and the whole mess clogs up the kidney filters over time.

With very few exceptions, if there is something in you that isn't you then you would be better off if it was dead.