Causes of High Histamine and Some Effects

An interesting description of histadelia causes and effects at Roo's Clues: http://roosclues.blogspot.com/2010/03/overview-of-high-histamine-also-called.html

Aside from under-methylation [a critical factor, determining how well histamine can be eliminated], Roo lists the following
conditions which can increase histamine:
  Adrenal fatigue.
  Damage to mast cells, e.g., by mercury.
  Chronic and excess exposure to allergens.
  Auto-immune conditions and chronic inflammation.
  Gut flora imbalances. Yeast overgrowth (especially with sensitivity to the body's own yeast).
  Deficient amylase, diamine oxidase or certain other enzymes 

The site also notes that high histamine can increase blood brain barrier permeability, heightening vulnerability to toxic injury, and facilitating viral infection of the brain.

For more information on histadelia, see my book, Natural Healing for Bipolar Disorder.