To evaluate the inter-arm and inter-period reproducibility of the dermal blood flow response induced by a skin prick of histamine, subjects will receive histamine (10 mg/ml) and negative control skin pricks on the volar surface of both forearms during two subsequent study visits to allow an intra-individual comparison. Changes in dermal blood flow will be measured during the hour after the skin pricks with laser Doppler and/or laser speckle contrast imaging.

Inclusion Criteria:

• Subject is a white male ≥ 18 and ≤ 45 years of age at the time of screening
• Subject is a nonsmoker for at least 6 months prior to the study start
• Subject has a body mass index ≥ 18 and ≤ 30 kg/m2
• Subject has a clear increase in histamine-induced dermal blood flow
• Subject is judged to be in good health on the basis of medical history, physical examination and vital signs
• Subject understands the procedures and agrees to comply with them for the entire length of the study by giving written informed consent

Exclusion Criteria:

• Subject has excessive hair growth on the volar surface of the forearm
• Subject has a past or present history of diffuse dermatological conditions including eczema, scleroderma, psoriasis, urticaria, dermatographism and dermatitis
• Subject has any abnormality on the skin of the forearm, possibly interfering with the study assessments including tattoos, keloids, tumors, ulcers, burns, flaps and grafts
• Subject cannot avoid excessive tanning (any exposure to sunlight or a tanning bed which would cause a sunburn reaction) throughout the study and cannot cover the forearms 24 hours before and after each study visit
• Subject currently uses lotions, oils, depilatory preparations, makeup or other topical treatments on the arms and on a regular basis which cannot be discontinued for the duration of the study; subject has used any topical treatments within 7 days of the start of the study
• Subject has a past or present history of (symptomatic) asthma
• Subject has a history of severe allergic reactions to food or drugs or adverse experiences of a serious nature related to the administration of either a marketed or investigational drug, including histamine
• Subject currently uses any prescription or nonprescription drugs on a regular basis which cannot be discontinued for the duration of the study; subject has used any prescription or nonprescription medication within 14 days of the start of the study
• Subject is a habitual and heavy consumer of coffee or caffeinated beverages (more than approximately 4 cups of tea, coffee or cola per day) at the time of the study. Subjects who have reduced their consumption to ≤ 4 cups per day at least 1 week prior to enrollment may participate. Subjects who cannot refrain from caffeinated beverages 24 hours before the study visit;
• Subject is unable to refrain from drinking alcohol 24 hours prior to histamine application, is currently a regular user (including "recreational use") of any illicit drugs, or has a history of drug (including alcohol) abuse. A drug screen will include amphetamines/ methamphetamines, methylenedioxymethamphetamine, benzodiazepines, barbiturates, cocaine, cannabis, tricyclic antidepressants, methadone and opiates
• Subject cannot refrain from being around second hand smoke 24 hours prior to histamine application or uses nicotine-containing products. Ex-smokers should have ceased smoking at least 6 months prior to screening
• Subject has any of the following vital sign measurements at screening: heart rate ≤ 40 or ≥ 100 beats/min, diastolic blood pressure ≤ 50 or ≥ 89 mmHg and/or systolic Blood Pressure ≤ 90 or ≥ 139 mmHg
• Subject has been involved in testing an investigational drug in another clinical study within the last 4 weeks or 5 half-lives
• Subject has evidence of a clinically significant active infection, fever of 38°C or above at the time of the study visits
• Subject has a history or current cardiovascular, respiratory, hepatic, renal, gastrointestinal or neurological disorders capable of significantly altering the absorption, metabolism or elimination of drugs
• Subject is in a situation or has a condition, which, in the opinion of the investigator, may interfere with safe and optimal participation in the study
• Subject has a history of any illness or disorder, which, in the opinion of the investigator, might confound the results of the study.