the type IV (late-phase) symptoms of chronic nasal congestion 24 to 48 hours after allergen exposure.

• Some watery discharge is also produced by the late-phase allergic response.

Priming Effect

• The allergen dose needed to elicit a response decreases with multiple exposures to that allergen.

• This means patients will often start to experience symptoms very early in the pollen season when levels are still low.

• Over the season more inflammatory cells migrate to the submucosa.

• Sensory nerves are stimulated by inflammatory cell products to lower their threshold for activation.

• Eosinophil-derived nerve growth factor increases sensory nerve expression of transient receptor potential vanilloid 1 receptor; this is the receptor through which histamine largely works through to induce neural symptoms.

• Towards the end of the pollen season, a lower level of allergen will cause more symptoms in the presence of activated sensory nerves and more submucosal inflammatory cells.

• Patients will also still experience significant symptoms towards the end of the season when pollen levels are dropping.

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