What Makes Pine Pollen Allergy So Rare?

By Claire Williams
http://cgwilliams.wordpress.com/

Pinus pollen allergy is rare and clinically insignificant despite the fact that pine pollen is so prevalent (1). Many reports from all parts of the worldwide echo this same result, even though every study seems to have a few individuals which are indeed allergic to pine pollen. Biomedical experts have proposed the following hypotheses for this paradox of pine pollen allergy rarity (1):

Hypothesis 1 : The pine pollen grain (44 to 80 microns in diameter) is too large to penetrate the bloodstream via the human respiratory system.

Hypothesis 2: The protein content of pine pollen is too low

Hypothesis 3: The hydrophobic nature of the pollen grain’s outer covering or exine alters contact with its allergen proteins.

The first hypothesis can be discounted – so strike this one. See hypothesis 3 for a more refined version of a similar idea.

The second hypothesis is also not so well-developed. Why? Consider that a pine pollen grain is a mobile male gametophyte, an independent life form, wrapped in a spore wall supplied by its parent tree. As a life form, it has some protein content in its total of four or five cells as does every other pollen grain. Juniperus ashei is a conifer which causes a severe allergy, mountain cedar fever, and its pollen has similar levels of protein.

The third hypothesis is the most plausible but it too needs more explicit development. Oddly, it is true that a pine pollen grain does not burst upon its contact with watery solutions. Try this with a water glass and a hand-held 10X hand lens. Pine pollen has different protein (not more, not less) profiles compared to those conifers which do produce allerge-causing pollen. This hypothesis is a keeper, deserving of better hypotheses and closer studies using genomics and related -omics approaches.

Consider this limited but intriguing support, i.e. many Juniperus species which are more closely related to the offending species Juniperus ashei share its small pollen size and the same exine-bursting tactic. But DNA sequencing of the allergen protein in Juniperus ashei shows that the same sequence in relatives which do not have allergy-causing pollen have important codon changes including a premature stop codon (see citations from earlier blog post). The difference is hypothesized to be molecular, not morphological.

That’s my bet for the pine story too.

(1) Marcos C., FJ Rodriguez, I Luna, V Jato and R. Gonzalez. 2001. Pinus pollen aerobiology and clinical sensitization in northwest Spain. Annals of Allergy, Ashtma and Immunology 87: 39-42. Freeman G.L. 1993. Pine pollen allergy in northern Arizona. Ann. Allergy 70: 491-494. Rowe A. 1939. Pine` pollen allergy. J. Allergy 10: 377-378.