In a previous post, I mentioned for the first time that I have polycythemia vera, which is a myeloprolific neoplasm (MPN), which is a mutant stem cell in the bone marrow that results in the overproduction of any variety/combination of blood cells.
On of the very, very minor effects of this is pruritis - terrible itching. If you know me, you've seen me itching like I have fleas at times and it's embarrassing sometimes and annoying all the time. It is due to increased histamines, similar to an allergic reaction, due to increased Eosinophils and Basophils in the blood count.
This video is the best explanation I've found, so for you fellow MPN people, take a listen. Also, allergy sufferers, this will help you understand your reactions better: Dr. Susan Leclair.