The only documented explanation I was able to find for nicotine's effect on allergy suppression is that the initial "kick" is caused by stimulation of the adrenal gland, which in turn suppresses allergic symptoms (as in the use of an epinephrine injection for severe asthma or other allergic symptoms).
And there's this from Science Daily:
"Cigarette Smoke Can Prevent Allergies, Study Suggests
May 18, 2009 — Everyone knows that smoking can kill you, but new research suggests that it may help with your allergies. A study of mouse mast cells shows that cigarette smoke can prevent allergies by decreasing the reaction of immune cells to allergens.
Smoking can cause lung cancer, pulmonary disease, and can even affect how the body fights infections. Along with many harmful effects, smoking cigarettes has a surprising benefit: cigarettes can protect smokers from certain types of allergies. Now, a study recommended by Neil Thomson, a member of Faculty of 1000 Biology and leading expert in the field of respiratory medicine, demonstrates that cigarette smoke decreases the allergic response by inhibiting the activity of mast cells, the major players in the immune system's response to allergens.
Researchers at Utrecht University in the Netherlands found that treatment of mast cells with a cigarette smoke-infused solution prevented the release of inflammation-inducing proteins in response to allergens, without affecting other mast cell immune functions.
The mast cells used in the study were derived from mice, but it is likely that the same anti-allergy effect will hold true in humans. While taking up smoking to cure allergies is unwise, Thomson concludes that the findings presented in this study are "consistent with a dampening of allergic responses in smokers."
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