The specific aim of this study is to validate iontophoresis (with measurement of microvascular blood flow by laser Doppler flowimetry) in a pediatric cohort as a surrogate marker capable of reliably characterizing the normal physiologic response of the cutaneous microvasculature to histamine. In future studies laser Doppler flowimetry ultimately will be used to assess whether microvascular blood flow determined by a previously validated laser Doppler technique can be used as a reliable surrogate endpoint capable of functionally discriminating the effects of polymorphisms in the genes which are quantitatively important for histamine metabolism. The hypothesis to be proven is that histamine iontophoresis with measurement of microvascular blood flow by laser Doppler flowimetry can be used as a reliable marker to characterize the normal microvascular cutaneous response to histamine.

Inclusion Criteria:

diagnosed with allergic rhinitis by an attending Pediatrician or Allergist (diagnosis made on clinical grounds or by positive skin test or RAST to environmental allergens, trees, grass, weeds, mold, animal dander, cockroach, dust mite)

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Exclusion Criteria:

history or evidence of McCune Albright syndrome, immunodeficiency, mastocytosis, receipt of allergy shots, chronic skin disease, liver or kidney disease, cancer, neurological movement disorders, uncontrolled ADHD, evidence of eczema currently or in the past 2 years, history of anaphylaxis or anaphylactic like episode, evidence of pregnancy (by urinary hCG) or lactation, use of antihistamines 10 days prior to skin testing, use of systemic corticosteroids or tricyclic antidepressants 30 days prior to skin testing, presence of condition that would cause difficulty in adherence with study procedure.