The hypothesis to be proven is that histamine iontophoresis with measurement of microvascular blood flow by laser Doppler flowmetry can be used as a reliable marker to characterize the normal microvascular cutaneous response to histamine.

The specific aim of this study is to validate iontophoresis (with measurement of microvascular blood flow by laser Doppler flowmetry) 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 flowmetry 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 flowmetry can be used as a reliable marker to characterize the normal microvascular cutaneous response to histamine.

Inclusion Criteria:

-Established diagnosis of allergic rhinitis and/or allergic conjunctivitis, and/or allergic asthma either clinically (by presence of rhinitis and/or conjunctivitis with or without associated wheezing) and/or with a positive RAST/skin test to the following allergens: trees, grass, weeds, molds, cat, dog, dust mites and/or cockroach)

Exclusion Criteria:

• Previous history or laboratory evidence of McCune Albright syndrome, immunodeficiency, mastocytosis, receipt of immunomodulatory treatment, chronic conditions associated with abnormalities of the integument, hepatic or renal compromise, neoplastic disease, movement or neurologic disorders and uncontrolled ADHD
• Active atopic dermatitis at the site of the skin prick test or site where histamine iontophoresis will be performed, i.e. active atopic dermatitis on anterior forearms
• History of anaphylaxis or anaphylactic like episode
• Evidence of pregnancy (by urinary hCG) or lactation
• Use of drugs (within a specified time period) of agents capable of altering the response to histamine provocation (e.g., antihistamines, systemic corticosteroids, tricyclic antidepressants)
• Presence of any condition that, in the opinion of the investigator, would produce difficulty with adherence to study procedures.

Pediatric patients with allergic rhinitis