system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some allergies only affect certain parts of the body?

Many allergies seem to only affect the respiratory system or the digestive tract. Why can the same allergen be rubbed on skin in some cases and no reaction occurs, or post stomach not cause reactions throughout the rest of the digestive systems? 

I am assuming there is no reaction after leaving the stomach due to the acid denaturing the protein responsible, but why is this the case for other parts of the skin/body?
gpt
The antigens have to be bound to IgE and then end up activating immune cells (mast cells especially). That's just much easier to accomplish in the respiratory and digestive tract. Unbroken skin is an excellent barrier for entry, and the respiratory and gastrointestinal tract make up most of the rest of the barrier between "outside" and "inside" of the body (the gut lumen etc. is topologically outside the body).