It has lots of false positives and false negatives. It is actually a very simplistic system, if it was "smart enough" we wouldn't have diseases. In fact, a good number of diseases, especially chronic ones that millions or billions of people suffer from throughout their lives, are actually the immune system's fault. It is not a "smart" system. It is clever. Sometimes. Until it's not.

All cells are covered in little bumps called antigens. Your body contains antibodies that bind to antigens on foreign cells. Your white blood cells then only attack cells that have antibodies attached to them.

So it's not about good/bad, but really just about which cells are your own and which are foreign.

They don't go after everything foreign. They are produced with instructions by which they identify a threat.
It's their production source which provides the instructions and it's not that smart or we wouldn't have cases where the immune system attacks the body itself.

yes, in fact our moral reasoning comes from the function of white blood cells and their extreme xenophobia - it is an important lesson to learn that foreign = bad, white = good ... oh wait, hmmmmmmmmm

Not really. That's what common food allergies are. It's the body's inability to tell good from bad and going to war with something benign.

I thought it was that certain roaming immune cells are basically pretty interested in literally everything they encounter, but you're own cells have chemicals cues that signal they are ok. And as soon as a they doesn't get the ok signal, all hell breaks loose, signaling the home bade, calling for backup, etc.

But this can go wrong, with like bacteria or cancer trying to mimic the "ok" Signal.

If someone wants to add, please do, because although i'm a biologist i mostly remember that the immune system is way more complex than you'd think.

How do you explain autoimmune diseases then?

Despite all the caveats, red blood cells “discern bad from good” better than most government officials.

Allergies.

Nothing is perfect, no defense is 100%.

I don't think a dog in heat is a good analogy, seeing as immune cells attack invaders, not try to have reproduce with them. Anyway, others have given good answers.

Your own cells have protein signature directly dependent on the genetics. Everything that doesn't match is attacked

Being a foreigner doesn't automatically mean something is bad - just unfamiliar.