Link: Mast cell activation syndrome 101

Mast cell activation syndrome 101 by synecdochic. Amazing writeup. Small sample:

Mast cells are a part of your immune system that are present in connective tissue (which is why the disorder is very often comorbid with Ehlers-Danlos and the other conective tissue disorders). They store histamine and heparin, and release them when encountering certain environmental conditions and allergens, in order to help your immune system handle those triggers. MCAS happens when your mast cells are very bad at identifying environmental conditions and allergens they should freak out at, and instead they freak out ("degranulate") at literally fucking everything, oh my fucking god will y'all chill in there. It's not an allergy per se, because it's not an IgE-mediated immune response, but it's very similar and some of the symptoms are functionally identical.