system
Given the following post, respond as if you're commenting on Reddit:
human
Why can the lone star tick bite make you allergic to red meat, if that very same allergene your body starts reacting to has always been present in the meat itself?

So I've heard that getting bitten by the lone star tick will make you allergic to red meat, specifically to saccharide "alpha-gal", present in most mammals, except humans. I wonder why we don't develop this allergy by eating the meat itself. Wouldn't that sugar circulate the body and be detected as foreign anyway? I'm sure I'm missing an important mechanism here, I just can't figure out why the body is fine with it as long as it's exclusively eaten, but after a bite you can't eat it anymore. I'm really curious about this acquired allergy.
gpt
It doesn't happen when we eat alpha-gal because alpha-gal is not dangerous. The immune system confuses alpha-gal with the dangerous tick proteins after exposure to the tick proteins. 

Immune cells generally learn to attack proteins. A protein is shaped kinda like a puzzle piece. It has a shape on the outside where it can attach to things.. Immune cells attach to these puzzle pieces so they can't attach to anything else and do damage. That's how they "deactivate" foreign proteins, viruses, etc. When the immune system attacks a protein it creates memory immune cells that keep the shape that fits into that protein puzzle piece so that if the protein returns it can be identified by the immune system, and trigger an attack on that protein.

When ticks bite you some of the proteins & stuff from their recent meals (generally mammals) get into the bite. When the immune system shows up, in addition to the tick's proteins which are causing damage, another thing they find *at the site where damage is occurring* is a mammal saccaride called alpha-gal. They make memory cells to protect against alpha-gal *even though that is not what's causing the damage* - it's just present when & where the tick proteins are causing damage. The immune system is confused.

EDIT: alpha-gal is the only true food allergy triggered by a non-protein. It's not totally known why but the theory is it just *looks* a lot like a protein to some people's immune systems.