Don't have specific answer to your question, but I remember from Dr. Walsh's excellent book "Nutrient Power" that high histamine and pollen allergy is quite common if you are a so called "Undermethylator" (scarcity of methyl groups for critical biological processes). High homocysteine is another characteristic.
This article tries to connect it all - MTHFR, homocysteine, histamine, undermethylator.
http://mentalhealthdaily.com/2015/0...vs-overmethylation-causes-symptoms-treatments
Note that if you wanna try a "methylation protocol" to increase your available CH3 (methyl groups), that Dr. Walsh has found through 30 years of practice that "Undermethylators" (UM) are often intolerant of FOLATE and should use SAMe and Methionine (what about TMG?) instead - in addition to supplements like; B-6, Zn, Ca, Mg, antioxidants (Vitamins A, C, D, E + Selenium). He recommends UMs to avoid: Choline, Folate, DMAE, B-5, Mangan, Copper.
People with high histamine need to bring it down before starting methylation nutrient therapy by taking B-6 and Serine (PhosphatidylSerine?).
Read the book. One of the best I have read on the subject of biochemistry and methylation.
https://www.amazon.com/Nutrient-Power-Heal-Biochemistry-Brain/dp/1626361282