SummaryChanges in local skin blood flow after prick‐tests with histamine and allergen challenge were evaluated using laser doppler flowmetry. Two series of measurements were performed; each included 11 subjects with seasonal allergic rhinitis. In the first series vascular reactions were registered intermittently for a period of 6 hr. This was then repeated with additional registrations after 14 and 24 hr. Registrations were made in the skin close to where the test substances were applied which was in the area of the initial weal reaction. Pre‐loaded skin‐prick test needles were used for the histamine and allergen tests. Controls using ‘blank’ needles were also set on the same occasion. The control induced a transient increase in blood flow which had disappeared after 1 hr. After histamine challenge, the initial rapid increase in blood flow was followed by a slow return to baseline within 1 hr, and no further changes were noticed during the registration period. A different blood flow response was seen after the application of allergen. After an initial increase, the blood flow remained at this higher level for more than 6 hr. Thereafter a slow decrease towards baseline was seen within 24 hr. The pronounced difference between the histamine‐ and allergen‐induced responses in the later part of the registrations after similar initial peak responses indicates that actions other than an initial burst of released histamine are responsible for the changes in dermal blood flow observed after allergen. Furthermore, our results suggest that an allergen challenge induces a continuous change in blood flow for up to 24 hr, rather than a biphasic response, in only some subjects which might be suspected from the visual reappearance of redness and induration of the skin which characterizes a dermal late
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