Anaphylaxis
Anaphylaxis usually happens because the immune system over-reacts to an allergen.  The immune system releases mast cells and basophils, which are types of white blood cells.  The immune system sees the trigger as something bad and sends out white blood cells to kill it.  The mast cells and basophils attach to the allergen to try to kill it.  However, because the system is over-reacting, the mast cells and basophils explode and dump out chemicals that cause inflammation, like histamine.  Histamine causes many of the symptoms of anaphylaxis.  For example: